Banishment and plane shift work slightly differently, so I'll address them separately
For banishment, assuming that being swallowed hasn't blinded you, (preventing you from casting banishment which requires you to be able to see the target) it only sends one creature (the target) to another plane. No other creatures, including yourself, are affected per the targeting rules; there are no rules for combining multiple creatures into a single target other than swarm rules.
For plane shift, the spell specifies that you and up to eight willing creatures link hands in a circle (obviously if it's just you, that effectively leaves you holding your own hands, but let's not think about that). Assuming the creature who swallowed you is also a willing creature (strange scenario, but not impossible), what would be impossible (most likely) would be for you to link hands with them while swallowed.
The short version is that if the spell specifies one target creature, any creatures that target may be grappling or have swallowed will be left behind.
It's an interesting question. I think I would also probably rule that you were left behind, but there are several possible peculiarities to both interpretations. While I don't think it's necessary to answer them all to make a ruling and play the game, it is interesting to consider.
I will assume we are only talking about living creatures, so dead organic matter eaten (food) would always travel with the target. Otherwise, every banishment/planeshift/etc would leave a disgusting pile of part-digested matter splattered on the floor behind it.... Eeeewww!
If it is only the creature targeted which is banished, what of the bacteria living in their gut? What if they had a parasite? Have we just discovered a way to cure someone of worms/tapeworm/sleeping sickness? If not, why not? What about a target which was pregnant? Would the fetus be left behind? What damage would this do to the target and fetus?
If, instead, the creature who was eaten was also banished, what would the rules be about this? Would they need to be completely consumed (in the stomach or beyond), or would something in the mouth be banished, too? Could this be used to transport many creatures, by having them all "eaten" by a friendly dragon? Would they be returned if the banishment ended, and would the answer to this be affected by whether they had escaped from the creature's stomach?
I think that this is one of those cases where you can wave aside hard logic and hard rules and apply some "it's magic!" to make it work neatly.
• if it affects one creature it's fair to say that any creature swallowed by them wouldn't go with them unless they were willing (EG tapeworms, gut bacteria) • If it affects a pregnant creature then it would be pretty gross to leave a fetus behind - see above.
I love the idea of someone banishing a dragon and a gnome drops to the floor, dripping in stomach juices, and screams "I'm Alive!" before running off in crazed euphoria.
If it is only the creature targeted which is banished, what of the bacteria living in their gut? What if they had a parasite? Have we just discovered a way to cure someone of worms/tapeworm/sleeping sickness? If not, why not? What about a target which was pregnant? Would the fetus be left behind? What damage would this do to the target and fetus?
You are thinking about it too much, but let's fix it anyway.
Bacteria etc arent statted and are only considered a disease by the game. We can simply say that they don't count as creature for the purposes of spells or abilities. Fixed.
Pregnancy is different but just as easy. The fetus is simply considered part of the mother's body. Fixed.
Swallowed creatures are independent and autonomous from the swallowing creature. There is usually a way to escape from inside, so they shouldn't get forced into the plane shift.
Would you planeshift with it or you would the spell leave you behind?
Banishment and plane shift work slightly differently, so I'll address them separately
For banishment, assuming that being swallowed hasn't blinded you, (preventing you from casting banishment which requires you to be able to see the target) it only sends one creature (the target) to another plane. No other creatures, including yourself, are affected per the targeting rules; there are no rules for combining multiple creatures into a single target other than swarm rules.
For plane shift, the spell specifies that you and up to eight willing creatures link hands in a circle (obviously if it's just you, that effectively leaves you holding your own hands, but let's not think about that). Assuming the creature who swallowed you is also a willing creature (strange scenario, but not impossible), what would be impossible (most likely) would be for you to link hands with them while swallowed.
The short version is that if the spell specifies one target creature, any creatures that target may be grappling or have swallowed will be left behind.
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It's an interesting question. I think I would also probably rule that you were left behind, but there are several possible peculiarities to both interpretations. While I don't think it's necessary to answer them all to make a ruling and play the game, it is interesting to consider.
I will assume we are only talking about living creatures, so dead organic matter eaten (food) would always travel with the target. Otherwise, every banishment/planeshift/etc would leave a disgusting pile of part-digested matter splattered on the floor behind it.... Eeeewww!
If it is only the creature targeted which is banished, what of the bacteria living in their gut? What if they had a parasite? Have we just discovered a way to cure someone of worms/tapeworm/sleeping sickness? If not, why not? What about a target which was pregnant? Would the fetus be left behind? What damage would this do to the target and fetus?
If, instead, the creature who was eaten was also banished, what would the rules be about this? Would they need to be completely consumed (in the stomach or beyond), or would something in the mouth be banished, too? Could this be used to transport many creatures, by having them all "eaten" by a friendly dragon? Would they be returned if the banishment ended, and would the answer to this be affected by whether they had escaped from the creature's stomach?
I think that this is one of those cases where you can wave aside hard logic and hard rules and apply some "it's magic!" to make it work neatly.
• if it affects one creature it's fair to say that any creature swallowed by them wouldn't go with them unless they were willing (EG tapeworms, gut bacteria)
• If it affects a pregnant creature then it would be pretty gross to leave a fetus behind - see above.
I love the idea of someone banishing a dragon and a gnome drops to the floor, dripping in stomach juices, and screams "I'm Alive!" before running off in crazed euphoria.
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You are thinking about it too much, but let's fix it anyway.
Bacteria etc arent statted and are only considered a disease by the game. We can simply say that they don't count as creature for the purposes of spells or abilities. Fixed.
Pregnancy is different but just as easy. The fetus is simply considered part of the mother's body. Fixed.
Swallowed creatures are independent and autonomous from the swallowing creature. There is usually a way to escape from inside, so they shouldn't get forced into the plane shift.