I have to admit. While I am quite serious about holding my ground in this conversation, I can't help finding it hilarious to first being told by my objectors that the unseen rule doesn't apply to swallowed creatures because the situation doesn't feel sensible, just for them to justify their point of view by claiming that creatures basically have eyes inside their stomachs...
Different people. I never specified anything about what feels sensible, just what the rules actually say.
Fair point. And yes, like you said, many swallowing creatures have Blindsight. The Tarrasque is a well-known example. Giant Frogs don't, though. But I don't think matters. Even if we don't consider the fact that swallow actions often explicitly give Total Cover to swallowed creatures, it can easily be inferred by the fact that there are several layers of fully encasing solid surfaces between a creature's own stomach and the outside of its body (I can't believe I even need to point that out)... And as we all know, Blindsight doesn't see through Total Cover.
I have to admit. While I am quite serious about holding my ground in this conversation, I can't help finding it hilarious to first being told by my objectors that the unseen rule doesn't apply to swallowed creatures because the situation doesn't feel sensible, just for them to justify their point of view by claiming that creatures basically have eyes inside their stomachs...
Different people. I never specified anything about what feels sensible, just what the rules actually say.
The rules actually say that an Unseen Attacker has advantage.
As for whether or not an attacker is unseen -- that is generally determined by the DM as you've pointed out. The DM has to make a ruling about that based on the specific situation that is occurring in the game and there are plenty of edge cases that exist. The DM will use various rules such as Line of Sight, Heavily Obscured areas, and so on for guidance and then ultimately the DM just has to make a ruling about whether or not a creature can be seen. Whether or not there is an actual rule about whether or not a creature can see inside of its own stomach is beside the point because of course it cannot. If a DM claimed to be making a RAW-based ruling and tried to then justify the ruling by saying that the creature can see inside of its own stomach, then I would strenuously object. If a DM instead admitted that he would prefer to make a non-RAW ruling for this situation such that applying both advantage and disadvantage remains disadvantage instead of canceling out in this case and asked us to just go with it then of course that's fine. But "how would I rule it" is a different conversation than "what is the RAW". The OP was specifically looking for a RAW answer.
And as we all know, Blindsight doesn't see through Total Cover.
Yeah, but being swallowed doesn't actually provide total cover against the swallowing monster (I think I've only once used a swallowing monster, and I don't think the character who got swallowed ever tried to attack the worm, so...).
Something that came up a while ago during a campaign with a particularly hostile DM I used to have: one player had been swallowed by another creature, which gave them Total Cover against the outside, as well as the Restrained and Blinded condition, which meant they had disadvantage on their attacks against the creature that had swallowed them.
I argued that because they were inside their stomach, the creature couldn't possibly see them
RAI: if the writer's intended that they likely would've mentioned it (instead of just mentioning the two conditions that grant disadvantage). It seems like it didn't occur to them.
Incidentally, and this is a house rule, I generally consider touch to provide the effect of sight, so you can cast spells that require sight if you're touching (outside of swallow, usually only applies to grapples). This does help a spellcaster who is trapped, as it means that you can, for example, magic missile the creature.
I've seen a couple of people argue that the Total Cover mentioned in swallow actions doesn't apply to the swallowing creature. If that was true, the creature could still reach you with their bites/tail/claws while you are inside them, which is ridiculous.
This Total Cover is given to make clear that while inside a swallowing creature, you are safe from their other attacks. If might seem [strange] to say it like "a creature isn't outside itself", but it sounds just as [strange] to imply that it can reach and see inside itself.
If you agree that it can't target or reach inside itself, then you've just described Total Cover.
I've seen a couple of people argue that the Total Cover mentioned in swallow actions doesn't apply to the swallowing creature. If that was true, the creature could still reach you with their bites/tail/claws while you are inside them, which is ridiculous.
It's also RAW. You're free to think it's dumb, but you can't pick and choose -- you can dislike the way the DM ruled, but it's no more wrong than the way you want it to work.
It's not the only RAW. It all comes from an interpretation of what "outside" means in that context. And RAW, every part of the creature that is not their stomach is outside their stomach. That includes tail, jaws, and claws. That's also RAW. That's the problem with RAW, there are often several ways of reading the exact same sentence. Just because we're having a RAW discussion, doesn't mean that only 1 reading is valid.
It's not the only RAW. It all comes from an interpretation of what "outside" means in that context. And RAW, every part of the creature that is not their stomach is outside their stomach.
Yeah, but it doesn't say "outside the stomach". It says "outside the creature".
RAI: if the writer's intended that they likely would've mentioned it (instead of just mentioning the two conditions that grant disadvantage). It seems like it didn't occur to them.
One thing that 2024 has made clear is that the designers wanted less text, not more. So arguments like "if the devs wanted this, they would have written it" are rarely valid.
2024 Dual Wielder feat if one of my favorite examples of that. This feat is pretty much useless on its own, but shines when used with Nick. I've seen other DMs argue that you can't get an extra attack with you "main" weapon by combining a Nick "off-hand" weapon with this feat because even if it's RAW, this combination was likely not intended, and it's an exploit, and so on. But considering that this is the only case where 2024 Dual Wielder is actually beneficial, it's clear to me that this is very much the intent behind the rewriting of this feat in 2024, even though the feat itself doesn't mention Nick in any way. And don't get me started on all the texts that has been removed from 2024 spells that effectively don't change anything to what these spells do. My point is: just because the devs didn't explicitly mention something, doesn't mean they didn't think of it or didn't intend for it.
The Unseen Attacker and Target rule has always been a black sheep in the games I've played. It's the reason why nobody attacks with disadvantage when attacking to or from inside a Darkness spell or a Fog Cloud (without special senses). Yes, you can't see where your attacks go, but your enemies to don't them coming. That's why the disadvantage cancels out. I feel like every DM hates this rule and jumps at the first opportunity to ignore it, but it's important for the balance of the game. Being swallowed is already a shitty enough situation for the player that on top of that you'd have them miss 80% of their attacks as they're trying to get out. Insisting that being they're hindered from being surrounded by the monster's flesh but ignoring the fact that it also makes it pretty much impossible to miss your target. That's not a fair way to rule the game.
Being swallowed is already a shitty enough situation for the player that on top of that you'd have them miss 80% of their attacks as they're trying to get out. Insisting that being they're hindered from being surrounded by the monster's flesh but ignoring the fact that it also makes it pretty much impossible to miss your target. That's not a fair way to rule the game.
Swallow is by no means a shitty situation; most of the time swallow whole is a lousy power that the monster should probably not use.
This is another one of those discussions where the concept being discussed doesn't need to be written in the rules because it's just part of the default assumptions that need to exist as a framework that the rules are built upon. Like, gravity exists by default and functions similar to our real world. Or that rain is a substance that is mainly water as opposed to, say, cats and dogs. So there might be a rule which discusses how the general combat rules might be altered in the presence of rain, but that only makes any sense if we can all agree on what the term "rain" generally means, for example. That's part of the framework to use so that actual rules can be written.
In this case, we should assume that by default creatures cannot see through solid objects. From there we can create rules such as the Line of Sight rule where if such objects are arranged in a certain way then we may or may not be able to see into a certain space. But that rule is written based on the above assumption that creatures cannot see through certain things. For the task in question, we do not have to find a written rule which fits exactly to our scenario of attempting to see into our own stomach. We should already just assume that the rules assume that we all agree that by default a creature cannot see through solid objects.
Now, perhaps a creature might have something written into their Stat block that creates a Trait. Something like:
X-ray vision. The [ whatever creature ] can see through solid objects.
In that case, a specific rule is created that alters our assumptions. Another example is the spell description for Reverse Gravity. There aren't really any general rules for gravity -- those are assumed. But that spell explicitly causes gravity to behave contrary to expectations.
In this discussion, there isn't any reason to believe that the swallowing creature can see within its own stomach. By default, it should be assumed that it cannot, since by default creatures cannot see through solid objects. Therefore, the swallowed creature is Unseen.
This is another one of those discussions where the concept being discussed doesn't need to be written in the rules because it's just part of the default assumptions that need to exist as a framework that the rules are built upon. Like, gravity exists by default and functions similar to our real world. Or that rain is a substance that is mainly water as opposed to, say, cats and dogs. So there might be a rule which discusses how the general combat rules might be altered in the presence of rain, but that only makes any sense if we can all agree on what the term "rain" generally means, for example. That's part of the framework to use so that actual rules can be written.
In this case, we should assume that by default creatures cannot see through solid objects. From there we can create rules such as the Line of Sight rule where if such objects are arranged in a certain way then we may or may not be able to see into a certain space. But that rule is written based on the above assumption that creatures cannot see through certain things. For the task in question, we do not have to find a written rule which fits exactly to our scenario of attempting to see into our own stomach. We should already just assume that the rules assume that we all agree that by default a creature cannot see through solid objects.
Now, perhaps a creature might have something written into their Stat block that creates a Trait. Something like:
X-ray vision. The [ whatever creature ] can see through solid objects.
In that case, a specific rule is created that alters our assumptions. Another example is the spell description for Reverse Gravity. There aren't really any general rules for gravity -- those are assumed. But that spell explicitly causes gravity to behave contrary to expectations.
In this discussion, there isn't any reason to believe that the swallowing creature can see within its own stomach. By default, it should be assumed that it cannot, since by default creatures cannot see through solid objects. Therefore, the swallowed creature is Unseen.
I agree with you that a strict reading of RAW it would be reasonable to impose the Unseen Attacker effects to the situation, giving the swallowed creature a straight roll on attacks from within the stomach (Adv and Disadv canceling out).
However, your comment discusses "baseline assumptions" that we have about the world that don't need to be spelled out in the rules. Rain is some amount of water falling from the sky as we experience it in our own world. Gravity pulls on all objects and creatures equally towards the "earth". One cannot normally see through solid objects, and we have an understanding of what a "solid object" (or "opaque", I suppose) would normally be.
However this same logic can be used against the argument that the swallowed individual should get Advantage because of being unseen. Because we can, based on our experiences in reality, understand WHY an Unseen Attacker is granted Advantage through the rules: because with the attacker being unseen, the creature being attacked has a more limited ability to appropriately defend itself (either by dodging, holding up a shield, etc).
You hide in a deer stand and become an Unseen Attacker because it is easier to hit a deer with an arrow if it is not alerted to your presence and not able to react to the arrow as quickly. We can assume, just as we assume how gravity and rain work in D&D, that this is how being Unseen works to grant Advantage. And so logically, in a situation in which you don't get the same advantage of being unseen (making your target less able to predict your attack, and thus more susceptible to being hit by it), such as by being swallowed by that creature, that the Advantage wouldn't apply.
Now you might say "Yes, but the creature IS less likely to dodge your attack, because you are inside of it! How could it dodge or anticipate your attack!" and I get the argument, however that really wouldn't come from being "unseen". That isn't the case that the target has a limited ability to avoid your attack due to not knowing where the attack is coming from or seeing the attack, but much more like an attack made against a Restrained creature. You get advantage on a restrained creature because they physically cannot (or have limited ability to) anticipate and/or move out of the way/block the attack. That is much more similar to what is actually going on with attacking from inside of a creature.
At any rate, I think this is one of those situations where no one is wrong. There are arguments for applying strict RAW to the situation, and arguments for why the strict RAW really doesn't apply in edge cases like this. In any case, this is very much down to the DM, so in my opinion the OP stating what the DM should have done is incorrect.
. . . we can, based on our experiences in reality, understand WHY an Unseen Attacker is granted Advantage through the rules: because with the attacker being unseen, the creature being attacked has a more limited ability to appropriately defend itself (either by dodging, holding up a shield, etc).
You hide in a deer stand and become an Unseen Attacker because it is easier to hit a deer with an arrow if it is not alerted to your presence and not able to react to the arrow as quickly . . . And so logically, in a situation in which you don't get the same advantage of being unseen (making your target less able to predict your attack, and thus more susceptible to being hit by it), such as by being swallowed by that creature, that the Advantage wouldn't apply.
Keep in mind that being Unseen in D&D is not the same as being hidden. Being hidden implies that your position is unknown because you are not being perceived by any senses at all (you are unseen and unheard and unfelt, etc.). But you can be an Unseen Attacker even when your enemy can hear you clearly enough that your enemy knows exactly where you are. You still get advantage on attack rolls in those cases. For example, if I am invisible (from a Greater Invisibility spell) and I am in melee combat with you, if I attempt to circle around behind you then according to the rules you can simply track my movements with your hearing (because I am not actually hidden) and when I stab at you from a different square you still know exactly where it's coming from. But that doesn't matter. Because you cannot actually see me when I make my attack, I get advantage on that attack roll.
So, the idea that this is "a situation in which you don't get the same advantage of being unseen" . . . why not? Sure, the enemy might be using one of its other senses, such as the sense of touch, to track exactly where you are. But again, that doesn't matter for being an Unseen Attacker.
Note that I have not even bothered to chime in with my own take on how I would rule this situation or how I think it should be ruled in a real game. The OP was asking specifically for a RAW ruling. Once we know what the RAW ruling would be, if we intend to deviate from that I think that in general it's helpful if everyone in the group is on board with that. It's even better if your ruling causes everyone in the group to have more fun. But such alternate rulings are really a different discussion.
In this discussion, there isn't any reason to believe that the swallowing creature can see within its own stomach. By default, it should be assumed that it cannot, since by default creatures cannot see through solid objects. Therefore, the swallowed creature is Unseen.
The reason to think that the swallowing creature can see is that the swallow ability states what it does, and it prevents any other creature from seeing the swallowed creature. I would note that they would not specific that the swallowed creature is blinded if blocking vision was seen as inherent in being swallowed, because the swallowed creature generally is not blinded, they're just unable to see anything outside of the swallowing creature.
1) Is a swallowed character considered an "unseen attacker," according to the written rules? Yes, that's a logical reading of the rules.*
2) Is the DM wrong to rule that a swallowed character is an exception to the unseen attacker rule? Not at all. Deciding whether a given rule applies to the current situation, is something the DM is expected, and required, to do.
If you want to pursue it with your DM further, you might bring it up with the other players at the table, just to find out which way they think that should go. If the other players at your table think that a swallowed character shouldn't have an attack penalty compared to outside attackers, that might make a difference to your DM.
* Also, just as a technicality: you have 2 sources of disadvantage inside the creature, from being both blinded and restrained, but the rules explicitly mention that advantage and disadvantage cancel out, even if you have multiple sources of disadvantage and only 1 source of advantage (or vice versa). So again, yes, going just by the words on the page, you would have neither advantage nor disadvantage once all the factors are accounted for. But that's also assuming your DM doesn't have a house rule for situations of mismatched advantage and disadvantage (although I realize that's a separate topic).
. . . we can, based on our experiences in reality, understand WHY an Unseen Attacker is granted Advantage through the rules: because with the attacker being unseen, the creature being attacked has a more limited ability to appropriately defend itself (either by dodging, holding up a shield, etc).
You hide in a deer stand and become an Unseen Attacker because it is easier to hit a deer with an arrow if it is not alerted to your presence and not able to react to the arrow as quickly . . . And so logically, in a situation in which you don't get the same advantage of being unseen (making your target less able to predict your attack, and thus more susceptible to being hit by it), such as by being swallowed by that creature, that the Advantage wouldn't apply.
Keep in mind that being Unseen in D&D is not the same as being hidden. Being hidden implies that your position is unknown because you are not being perceived by any senses at all (you are unseen and unheard and unfelt, etc.). But you can be an Unseen Attacker even when your enemy can hear you clearly enough that your enemy knows exactly where you are. You still get advantage on attack rolls in those cases. For example, if I am invisible (from a Greater Invisibility spell) and I am in melee combat with you, if I attempt to circle around behind you then according to the rules you can simply track my movements with your hearing (because I am not actually hidden) and when I stab at you from a different square you still know exactly where it's coming from. But that doesn't matter. Because you cannot actually see me when I make my attack, I get advantage on that attack roll.
So, the idea that this is "a situation in which you don't get the same advantage of being unseen" . . . why not? Sure, the enemy might be using one of its other senses, such as the sense of touch, to track exactly where you are. But again, that doesn't matter for being an Unseen Attacker.
Note that I have not even bothered to chime in with my own take on how I would rule this situation or how I think it should be ruled in a real game. The OP was asking specifically for a RAW ruling. Once we know what the RAW ruling would be, if we intend to deviate from that I think that in general it's helpful if everyone in the group is on board with that. It's even better if your ruling causes everyone in the group to have more fun. But such alternate rulings are really a different discussion.
Correct with the "Unseen is not the same as hidden", however that's not really what my point was. The deer example was just one way to think of it, but I stated we assume the reason you get advantage by being Unseen is "because with the attacker being unseen, the creature being attacked has a more limited ability to appropriately defend itself (either by dodging, holding up a shield, etc)."
This is true in your example with the Greater Invisibility scenario. The reason you get advantage is because the creature is limited in defending itself through dodging or holding up a shield because they cannot see the attack coming. Even if they know you are there, exactly where you are, they don't know if you are attacking high with an axe or low with a dagger. But this is not true for a creature within another creature.
Again, strict reading of RAW the Adv and Disadv cancel and the swallowed creature attacks with straight rolls. However the OP seemed to be suggesting that the DM is hostile towards the players and "rules however they want", and seemed to imply that the DM was ruling this way as some kind of "gotcha" or bad-faith attempt to be overly antagonistic to the players. My point has been that it can reasonably be ruled either way. It has always been the stance of the devs that the rules are not 100% strict and do not always work explicitly as stated in every edge-case scenario that might arise. If we always read the rules as being 100% strict and unbendable, then creatures could not see the moon at night and a Level 1 Grung might die from their natural jumping ability.
"A swallowed creature has the Restrained condition, has Total Cover against attacks and other effects outside the kraken, and takes 24 (7d6) Acid damage at the start of each of its turns."
Different people. I never specified anything about what feels sensible, just what the rules actually say.
Fair point. And yes, like you said, many swallowing creatures have Blindsight. The Tarrasque is a well-known example. Giant Frogs don't, though. But I don't think matters. Even if we don't consider the fact that swallow actions often explicitly give Total Cover to swallowed creatures, it can easily be inferred by the fact that there are several layers of fully encasing solid surfaces between a creature's own stomach and the outside of its body (I can't believe I even need to point that out)... And as we all know, Blindsight doesn't see through Total Cover.
The rules actually say that an Unseen Attacker has advantage.
As for whether or not an attacker is unseen -- that is generally determined by the DM as you've pointed out. The DM has to make a ruling about that based on the specific situation that is occurring in the game and there are plenty of edge cases that exist. The DM will use various rules such as Line of Sight, Heavily Obscured areas, and so on for guidance and then ultimately the DM just has to make a ruling about whether or not a creature can be seen. Whether or not there is an actual rule about whether or not a creature can see inside of its own stomach is beside the point because of course it cannot. If a DM claimed to be making a RAW-based ruling and tried to then justify the ruling by saying that the creature can see inside of its own stomach, then I would strenuously object. If a DM instead admitted that he would prefer to make a non-RAW ruling for this situation such that applying both advantage and disadvantage remains disadvantage instead of canceling out in this case and asked us to just go with it then of course that's fine. But "how would I rule it" is a different conversation than "what is the RAW". The OP was specifically looking for a RAW answer.
Yeah, but being swallowed doesn't actually provide total cover against the swallowing monster (I think I've only once used a swallowing monster, and I don't think the character who got swallowed ever tried to attack the worm, so...).
RAW all the modifiers cancel out even if it doesn’t make the best sense.
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RAW: you are correct (discussed here).
RAI: if the writer's intended that they likely would've mentioned it (instead of just mentioning the two conditions that grant disadvantage). It seems like it didn't occur to them.
Either interpretation is fine.
Incidentally, and this is a house rule, I generally consider touch to provide the effect of sight, so you can cast spells that require sight if you're touching (outside of swallow, usually only applies to grapples). This does help a spellcaster who is trapped, as it means that you can, for example, magic missile the creature.
I've seen a couple of people argue that the Total Cover mentioned in swallow actions doesn't apply to the swallowing creature. If that was true, the creature could still reach you with their bites/tail/claws while you are inside them, which is ridiculous.
This Total Cover is given to make clear that while inside a swallowing creature, you are safe from their other attacks. If might seem [strange] to say it like "a creature isn't outside itself", but it sounds just as [strange] to imply that it can reach and see inside itself.
If you agree that it can't target or reach inside itself, then you've just described Total Cover.
It's also RAW. You're free to think it's dumb, but you can't pick and choose -- you can dislike the way the DM ruled, but it's no more wrong than the way you want it to work.
It's not the only RAW. It all comes from an interpretation of what "outside" means in that context. And RAW, every part of the creature that is not their stomach is outside their stomach. That includes tail, jaws, and claws. That's also RAW. That's the problem with RAW, there are often several ways of reading the exact same sentence. Just because we're having a RAW discussion, doesn't mean that only 1 reading is valid.
Yeah, but it doesn't say "outside the stomach". It says "outside the creature".
The thing is that, even RAI, I don't think it holds up...
One thing that 2024 has made clear is that the designers wanted less text, not more. So arguments like "if the devs wanted this, they would have written it" are rarely valid.
2024 Dual Wielder feat if one of my favorite examples of that. This feat is pretty much useless on its own, but shines when used with Nick. I've seen other DMs argue that you can't get an extra attack with you "main" weapon by combining a Nick "off-hand" weapon with this feat because even if it's RAW, this combination was likely not intended, and it's an exploit, and so on. But considering that this is the only case where 2024 Dual Wielder is actually beneficial, it's clear to me that this is very much the intent behind the rewriting of this feat in 2024, even though the feat itself doesn't mention Nick in any way. And don't get me started on all the texts that has been removed from 2024 spells that effectively don't change anything to what these spells do. My point is: just because the devs didn't explicitly mention something, doesn't mean they didn't think of it or didn't intend for it.
The Unseen Attacker and Target rule has always been a black sheep in the games I've played. It's the reason why nobody attacks with disadvantage when attacking to or from inside a Darkness spell or a Fog Cloud (without special senses). Yes, you can't see where your attacks go, but your enemies to don't them coming. That's why the disadvantage cancels out.
I feel like every DM hates this rule and jumps at the first opportunity to ignore it, but it's important for the balance of the game.
Being swallowed is already a shitty enough situation for the player that on top of that you'd have them miss 80% of their attacks as they're trying to get out. Insisting that being they're hindered from being surrounded by the monster's flesh but ignoring the fact that it also makes it pretty much impossible to miss your target. That's not a fair way to rule the game.
Swallow is by no means a shitty situation; most of the time swallow whole is a lousy power that the monster should probably not use.
This is another one of those discussions where the concept being discussed doesn't need to be written in the rules because it's just part of the default assumptions that need to exist as a framework that the rules are built upon. Like, gravity exists by default and functions similar to our real world. Or that rain is a substance that is mainly water as opposed to, say, cats and dogs. So there might be a rule which discusses how the general combat rules might be altered in the presence of rain, but that only makes any sense if we can all agree on what the term "rain" generally means, for example. That's part of the framework to use so that actual rules can be written.
In this case, we should assume that by default creatures cannot see through solid objects. From there we can create rules such as the Line of Sight rule where if such objects are arranged in a certain way then we may or may not be able to see into a certain space. But that rule is written based on the above assumption that creatures cannot see through certain things. For the task in question, we do not have to find a written rule which fits exactly to our scenario of attempting to see into our own stomach. We should already just assume that the rules assume that we all agree that by default a creature cannot see through solid objects.
Now, perhaps a creature might have something written into their Stat block that creates a Trait. Something like:
X-ray vision. The [ whatever creature ] can see through solid objects.
In that case, a specific rule is created that alters our assumptions. Another example is the spell description for Reverse Gravity. There aren't really any general rules for gravity -- those are assumed. But that spell explicitly causes gravity to behave contrary to expectations.
In this discussion, there isn't any reason to believe that the swallowing creature can see within its own stomach. By default, it should be assumed that it cannot, since by default creatures cannot see through solid objects. Therefore, the swallowed creature is Unseen.
I agree with you that a strict reading of RAW it would be reasonable to impose the Unseen Attacker effects to the situation, giving the swallowed creature a straight roll on attacks from within the stomach (Adv and Disadv canceling out).
However, your comment discusses "baseline assumptions" that we have about the world that don't need to be spelled out in the rules. Rain is some amount of water falling from the sky as we experience it in our own world. Gravity pulls on all objects and creatures equally towards the "earth". One cannot normally see through solid objects, and we have an understanding of what a "solid object" (or "opaque", I suppose) would normally be.
However this same logic can be used against the argument that the swallowed individual should get Advantage because of being unseen. Because we can, based on our experiences in reality, understand WHY an Unseen Attacker is granted Advantage through the rules: because with the attacker being unseen, the creature being attacked has a more limited ability to appropriately defend itself (either by dodging, holding up a shield, etc).
You hide in a deer stand and become an Unseen Attacker because it is easier to hit a deer with an arrow if it is not alerted to your presence and not able to react to the arrow as quickly. We can assume, just as we assume how gravity and rain work in D&D, that this is how being Unseen works to grant Advantage. And so logically, in a situation in which you don't get the same advantage of being unseen (making your target less able to predict your attack, and thus more susceptible to being hit by it), such as by being swallowed by that creature, that the Advantage wouldn't apply.
Now you might say "Yes, but the creature IS less likely to dodge your attack, because you are inside of it! How could it dodge or anticipate your attack!" and I get the argument, however that really wouldn't come from being "unseen". That isn't the case that the target has a limited ability to avoid your attack due to not knowing where the attack is coming from or seeing the attack, but much more like an attack made against a Restrained creature. You get advantage on a restrained creature because they physically cannot (or have limited ability to) anticipate and/or move out of the way/block the attack. That is much more similar to what is actually going on with attacking from inside of a creature.
At any rate, I think this is one of those situations where no one is wrong. There are arguments for applying strict RAW to the situation, and arguments for why the strict RAW really doesn't apply in edge cases like this. In any case, this is very much down to the DM, so in my opinion the OP stating what the DM should have done is incorrect.
Keep in mind that being Unseen in D&D is not the same as being hidden. Being hidden implies that your position is unknown because you are not being perceived by any senses at all (you are unseen and unheard and unfelt, etc.). But you can be an Unseen Attacker even when your enemy can hear you clearly enough that your enemy knows exactly where you are. You still get advantage on attack rolls in those cases. For example, if I am invisible (from a Greater Invisibility spell) and I am in melee combat with you, if I attempt to circle around behind you then according to the rules you can simply track my movements with your hearing (because I am not actually hidden) and when I stab at you from a different square you still know exactly where it's coming from. But that doesn't matter. Because you cannot actually see me when I make my attack, I get advantage on that attack roll.
So, the idea that this is "a situation in which you don't get the same advantage of being unseen" . . . why not? Sure, the enemy might be using one of its other senses, such as the sense of touch, to track exactly where you are. But again, that doesn't matter for being an Unseen Attacker.
Note that I have not even bothered to chime in with my own take on how I would rule this situation or how I think it should be ruled in a real game. The OP was asking specifically for a RAW ruling. Once we know what the RAW ruling would be, if we intend to deviate from that I think that in general it's helpful if everyone in the group is on board with that. It's even better if your ruling causes everyone in the group to have more fun. But such alternate rulings are really a different discussion.
The reason to think that the swallowing creature can see is that the swallow ability states what it does, and it prevents any other creature from seeing the swallowed creature. I would note that they would not specific that the swallowed creature is blinded if blocking vision was seen as inherent in being swallowed, because the swallowed creature generally is not blinded, they're just unable to see anything outside of the swallowing creature.
I see two separate questions here.
1) Is a swallowed character considered an "unseen attacker," according to the written rules? Yes, that's a logical reading of the rules.*
2) Is the DM wrong to rule that a swallowed character is an exception to the unseen attacker rule? Not at all. Deciding whether a given rule applies to the current situation, is something the DM is expected, and required, to do.
If you want to pursue it with your DM further, you might bring it up with the other players at the table, just to find out which way they think that should go. If the other players at your table think that a swallowed character shouldn't have an attack penalty compared to outside attackers, that might make a difference to your DM.
* Also, just as a technicality: you have 2 sources of disadvantage inside the creature, from being both blinded and restrained, but the rules explicitly mention that advantage and disadvantage cancel out, even if you have multiple sources of disadvantage and only 1 source of advantage (or vice versa). So again, yes, going just by the words on the page, you would have neither advantage nor disadvantage once all the factors are accounted for. But that's also assuming your DM doesn't have a house rule for situations of mismatched advantage and disadvantage (although I realize that's a separate topic).
Edit: minor rephrasing just for clarity.
Correct with the "Unseen is not the same as hidden", however that's not really what my point was. The deer example was just one way to think of it, but I stated we assume the reason you get advantage by being Unseen is "because with the attacker being unseen, the creature being attacked has a more limited ability to appropriately defend itself (either by dodging, holding up a shield, etc)."
This is true in your example with the Greater Invisibility scenario. The reason you get advantage is because the creature is limited in defending itself through dodging or holding up a shield because they cannot see the attack coming. Even if they know you are there, exactly where you are, they don't know if you are attacking high with an axe or low with a dagger. But this is not true for a creature within another creature.
Again, strict reading of RAW the Adv and Disadv cancel and the swallowed creature attacks with straight rolls. However the OP seemed to be suggesting that the DM is hostile towards the players and "rules however they want", and seemed to imply that the DM was ruling this way as some kind of "gotcha" or bad-faith attempt to be overly antagonistic to the players. My point has been that it can reasonably be ruled either way. It has always been the stance of the devs that the rules are not 100% strict and do not always work explicitly as stated in every edge-case scenario that might arise. If we always read the rules as being 100% strict and unbendable, then creatures could not see the moon at night and a Level 1 Grung might die from their natural jumping ability.
Swallowed isnt a standard condition. It would be something specific to a monsters star block, such as a kraken:
https://www.dndbeyond.com/monsters/5195097-kraken
"A swallowed creature has the Restrained condition, has Total Cover against attacks and other effects outside the kraken, and takes 24 (7d6) Acid damage at the start of each of its turns."
Restrained is a standard condition:
https://www.dndbeyond.com/sources/dnd/phb-2024/rules-glossary#RestrainedCondition
"Attacks Affected. Attack rolls against you have Advantage, and your attack rolls have Disadvantage."
Thats it. If the rules say what a feature does, thats all it does.
“Doubt is not a pleasant condition, but certainty is absurd.” — Voltaire