So we've finished the Beyond Icespire Peak trilogy and I'm home-brewing an add-on adventure to take the party to L20. I'm not familiar with the various monsters, so I'm asking if you have any suggestions for the final fight. Some background:
The party consists of an Evocation Wizard, a Valor Bard and a Champion Fighter (all straight up, no MCs).
The background of the adventure is that during the trilogy, the characters made a serious mistake and accidentally killed 7 innocents. They're now on a quest for seven special diamonds that will allow them to resurrect those dead and undo their mistake (I know that's not how the spell works, but we agreed on it to provide the plot motivation). We'll be working through several modified Candlekeep Mysteries that will get then researching the locations of the diamonds and going out to get them, plus a few homebrew quests.
My thought was that during the last quest, which is to travel into the Underdark, they'll accidentally awaken a great beast of some kind, which will go on a rampage and they have to, once again, clear up their mess, hopefully before it kills too many people.
Any suggestions as to what I could use? We can be fairly flexible with the lore. Or I could even have them awaken it earlier in the campaign but it's only at the end that the news of it catches up with them etc, so lore isn't a massive issue.
Thoughts?
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If you're not willing or able to to discuss in good faith, then don't be surprised if I don't respond, there are better things in life for me to do than humour you. This signature is that response.
I mean, the classic 'great beast that goes on a rampage' is the Tarrasque, though it's a pretty poorly designed boss so you might want something homebrew. There aren't a lot of critters that are a decent fight for even three level 20 PCs, mostly greatwyrm dragons and divine avatars, and even fewer that are particularly inclined towards mindless rampages, though you could have something gone mad.
Homebrew is definitely the way to go. While many Monsters above CR 20 have creative powers, they still need Buffs and not Nerfs. Additional hit points (sometimes a lot), more Resistances (including an extra Saving Throw Proficiency) and Immunities (don't forget Conditions), and here and there a buff to damage die (meaning 1 or even 2 extra die and/or increasing the die size (d8 to d10 by example). One thing I do also is look at Monsters of both the same type as well as close to the same CR to get a feeling there. And obviously Action Economy, which can include Speed and type of movement. For Legendary Actions and Resistances, I generally give 4 to Monsters of CR 22 to CR 26 or 27, and 5 for CR's above that.
I'd offer more information about the Campaign specifically but I'm not familiar with it, sorry.
Edit: Pantagruel is absolutely correct about the Tarrasque in that usage, and it is also a prime example of needing the Buffs I mentioned. The thing should basically be immune to every condition there is, but definitely not Prone, if only for the possibility of it falling down on a row of buildings, crushing everything beneath it. A prior edition suggested the Tarrasque as having a sort of aura like a gravity well that prevented creatures from flying too high off the ground around it, but others may feel this is a cheat or something. I do know that there are a lot of people that have homebrewed their own Tarrasque in the face of the 5e pale imitation.
The mindless rampage isn't set in stone... that's just a thought. The bit I want is that they cause a disaster and end up fighting a showdown type monster/enemy.
As far as the So far, I've got:
A homebrew quest to find a rare tome so that they can enter Candlekeep - involving vampires.
The Inner Alchemy - modified to be a book that contains the ritual.
An unwritten homebrew involving Dwarves.
The Canopic Being - The missing sage knows the location of the first diamond, the party has to track her down and in the process finds the diamond after defeating the Mummy Lord that is behind her disappearance.
An unwritten homebrew, involving a Dragon Cult that we met previously in the trilogy.
The Scrivener's Tale - pretty much as written, except the book allegedly holds the location of a second diamond (but doesn't, and they're cursed).
Another unwritten homebrew, in which they find the third diamond.
Alkazar's Appendix - The Nether Scroll holds the locations of the final four diamonds, involves an Adult Blue Dracolich.
Xanthoria - they're infected while looking for a diamond in a village. A Lichen Lich took the diamond and infected the villagers, with each part being a quest.
A homebrew where they have to raid a dragon's lair for a diamond (will be in the form of two quests).
A homebrew where they have to enter the Underdark to retrieve a diamond (will be in the form of two quests). I'm unsure as to whether to go with Drow, Enchaphalids, or just go for a creature feature.
The final quest is that they've someone awakened or triggered a serious threat. Hopefully I can work it so that defeating the threat gives a diamond - it's carrying or powered by the diamond or something - but otherwise there will be another homebrew quest before this so they can get the diamond, possibly involving the Mummy Lord from Canopic Being returning, depending on how they resolved the quest.
If you're not willing or able to to discuss in good faith, then don't be surprised if I don't respond, there are better things in life for me to do than humour you. This signature is that response.
If your PCs have no built their characters erratically, there is no single monster in any of the books that provides them more than about 3 turns of combat. They are almost all based on a single large (but not very large) AoE attack (or comparable) and then getting punched to death quite fast after that.
The Lichen Lich is a good example of this. Fire Storm is very weak (38 damage on a failed save) and then it basically has nothing to threaten the PCs with. It has an absolutely wretched 225 hit points. An optimised level 11 party of 4 are going to bring it down in 2 turns with ease. I really don't understand how it gets its CR18, I'd put it at about CR11.
So homebrew something. Homebrewing monsters is really good fun, and the more you do it, the better you'll get.
If not then I suggest one of the mythic dragons from Fizbans are probably your best bet.
The Lichen Lich is a good example of this. Fire Storm is very weak (38 damage on a failed save) and then it basically has nothing to threaten the PCs with. It has an absolutely wretched 225 hit points. An optimised level 11 party of 4 are going to bring it down in 2 turns with ease. I really don't understand how it gets its CR18, I'd put it at about CR11.
How is being dropped in two turns by an optimized level 11 party unexpectedly weak for a CR 18? AC 20/225 hp is a hair on the low side, but not dramatic, and even if never uses fire storm it has pretty substantial damage output (spamming poisonous touch is 122 dpr).
Panda-wat (I hate my username) is somehow convinced that he is objectively right about everything D&D related even though he obviously is not. Considering that, he'd probably make a great D&D youtuber.
"If I die, I can live with that." ~Luke Hart, the DM lair
The issues I've found with the Great Wyrms is that they are listed as being more challenging than an Ancient Dragon but they are obviously statistically much weaker. People counter that by pointing out they effectively have more hit points (but still, less combat power) than an Ancient, due to their Mythic status. But that itself doesn't answer why, if the higher challenge rating is due to resiliency, why the players should gain twice the experience of a CR 27 encounter. It shows the Great Wyrm is supposed to be equivalent to a back to back encounter but somehow the monster itself doesn't earn the CR of either the first or second half or that encounter.
The Lichen Lich is a good example of this. Fire Storm is very weak (38 damage on a failed save) and then it basically has nothing to threaten the PCs with. It has an absolutely wretched 225 hit points. An optimised level 11 party of 4 are going to bring it down in 2 turns with ease. I really don't understand how it gets its CR18, I'd put it at about CR11.
How is being dropped in two turns by an optimized level 11 party unexpectedly weak for a CR 18? AC 20/225 hp is a hair on the low side, but not dramatic, and even if never uses fire storm it has pretty substantial damage output (spamming poisonous touch is 122 dpr).
Honestly, two rounds for a single creature isn't that bad for a standard WotC monster, but that is still really sad. Most parties (and I checked with a Poll in the forums a while ago) prefer combats that last 4 to 5 rounds. Still. Boss monsters should always be legendary at a minimum, and standard WotC monsters clearly show they need a lot more hit points and better defenses, especially for "elite / lieutenant" Monsters, not to even speak of Bosses. Further these monsters deserve a lair that rewards their own maneuverability.
I've always wanted to use this badboy in a campaign, but haven't had the chance. It's number of attacks/AC/HP can be adjusted if they're too low for your group. Or you could just put three or four of them in a keep or sewer and have them patrol around. Might be something your players think they can handle, but when two or three of them show up in might be a different story. Live in cold, dank caves or dungeon complexes.
Monstrous melee attack creature that specializes in paralyzing and inflicting the frightened status. Heals itself and tramples its prey before raining blows. They can have nests of 3-4 cubs.
Honestly, two rounds for a single creature isn't that bad for a standard WotC monster, but that is still really sad. Most parties (and I checked with a Poll in the forums a while ago) prefer combats that last 4 to 5 rounds. Still. Boss monsters should always be legendary at a minimum, and standard WotC monsters clearly show they need a lot more hit points and better defenses, especially for "elite / lieutenant" Monsters, not to even speak of Bosses. Further these monsters deserve a lair that rewards their own maneuverability.
The core problem isn't actually with monsters, the core of the problem is with PCs. If two equal-CR monsters get into a brawl, the typical duration of that fight will be 4-5 rounds, which is abut right for a coinflip difficulty fight. If two equal level PCs get into a brawl, you're likely talking about 2 rounds.
The issues I've found with the Great Wyrms is that they are listed as being more challenging than an Ancient Dragon but they are obviously statistically much weaker. People counter that by pointing out they effectively have more hit points (but still, less combat power) than an Ancient, due to their Mythic status. But that itself doesn't answer why, if the higher challenge rating is due to resiliency, why the players should gain twice the experience of a CR 27 encounter. It shows the Great Wyrm is supposed to be equivalent to a back to back encounter but somehow the monster itself doesn't earn the CR of either the first or second half or that encounter.
I don't know what you mean by greatwyrms being weaker than ancient. For most dragons all of their attacks do more damage than the ancient counter part. There are a couple of the higher cr ancient dragons which do slightly more damage on their breath weapon but the greatwyrm gets a much larger targeting area and a higher save dc in return so it balances out. Its just a result of their basically being 3 greatwyrm stat blocks as opposed to 6+ ancient dragon stat blocks. Some of the ancient dragons have above typical damage on their breath.
Any way, the way you make monsters last longer if you want a single monster in dnd is damage gates. Basically things like the mythic trait that break the hit points up into sections so they they aren't all blown through at once. It can be different forms like Auriel, Mythic traits, Contingencies or polymorphs just something that caps player damage like phases in a boss fight. It functionally turns one monster into several monsters run consecutively.
We must be looking at totally different stat blocks in relation to the Great Wyrm and the Ancient Dragons. Ancient Dragons have more hit points (if you take away the Mythic aspect), their breath weapons definitely deal more damage, and their Bite / Claw / Claw attacks and damage are equal or close to. It is weird that in a number of cases the Wyrms deal force damage rather than the Elemental damage. Also, the force spears seem out of place for a Dragon, and weak. Honestly, there doesn't seem to have been much in the way of creative advancement especially, not to mention general combat effectiveness. If you like the Great Wyrms, obviously enjoy, but I'm not seeing how the math supports a more challenging monster (again, noting either the individual before and after incarnations of the Mythic, which are all that should matter seeing as how with Mythic it really just comes down to a back to back encounter).
Edit: Just double checked myself, and the Ancients aren't extraordinarily more powerful than the Great Wyrms (using the Red Dragon as reference) but they still are more powerful. For some reason I had thought the gap was bigger. Personally it's also weird to me that a CR 27 Legendary Monster doesn't have 4 Legendary Actions, but at least it gets 4 Legendary Resistances that also refresh on the Mythic transformation.
We must be looking at totally different stat blocks in relation to the Great Wyrm and the Ancient Dragons. Ancient Dragons have more hit points (if you take away the Mythic aspect), their breath weapons definitely deal more damage, and their Bite / Claw / Claw attacks and damage are equal or close to. It is weird that in a number of cases the Wyrms deal force damage rather than the Elemental damage. Also, the force spears seem out of place for a Dragon, and weak. Honestly, there doesn't seem to have been much in the way of creative advancement especially, not to mention general combat effectiveness. If you like the Great Wyrms, obviously enjoy, but I'm not seeing how the math supports a more challenging monster (again, noting either the individual before and after incarnations of the Mythic, which are all that should matter seeing as how with Mythic it really just comes down to a back to back encounter).
Edit: Just double checked myself, and the Ancients aren't extraordinarily more powerful than the Great Wyrms (using the Red Dragon as reference) but they still are more powerful. For some reason I had thought the gap was bigger. Personally it's also weird to me that a CR 27 Legendary Monster doesn't have 4 Legendary Actions, but at least it gets 4 Legendary Resistances that also refresh on the Mythic transformation.
Yeah the red dragon is one of the high CR outliers, the ancient green, white, blue and black all have lower health than the chromatic greatwyrm. However, even looking at the red dragon v chromatic greatwyrm, the greatwyrm has a stronger bite, stronger claws, stronger legendary actions, higher speed, higher dcs, higher hit modifier and it's attack inflict conditions which the ancient red's doesn't. The great wyrm is a significantly harder fight unless you're talking about insanely high damaging pcs
The issues I've found with the Great Wyrms is that they are listed as being more challenging than an Ancient Dragon but they are obviously statistically much weaker.
A mythic is functionally two CR x monsters in a row, so we can analyze either phase as a single monster. We'll start with defense
Ancient Red Dragon: AC 22, 546 hp, 3 legendary resistance, 4 proficient saves.
Red Greatwyrm: AC 22, 533 hp, 4 legendary resistance, 4 proficient saves, immunity to charm, frightened, poisoned.
Red Greatwyrm phase 2: AC 22, 450 hp, otherwise the same.
So, defensively ancient and greatwyrm phase 1 are about equal, which means we need to compensate by offensive CR. Looking at dpr math, in three rounds we expect
Ancient Red Dragon: Breath 2 targets (182), Bite x2 (70), Claw x4 (68), Tail x9 (171), total 491. Applies fear with frightful presence.
Red Greatwyrm: Breath 2 targets (156), Bite x2 (68), Claw or Tail x13 (247), total 471. Slightly higher attack bonus and save DC. Applies restrained with claws.
Red Greatwyrm P2: replace claws with bites (+135 damage)
The greatwyrm also has a lot more breath weapon range, but that's not officially part of CR math, so I have to agree they're overall about the same CR. My suspicion is that dragons in the monster manual are somewhat overpowered for their listed CR, and Fizban's actually used the official rules for CR.
@Pantagruel666 I actually should review some of the newer monsters under the proposed rating system of the DMG. I remember when it was first put out, people all thought it was BS because none of the MM entries matched up with their own CR, the general community assumption being that it was just a defensive gimmick to pretend there was an actual system when there wasn't one. That's what I remember anyway.
A couple of options come to mind for an underdark based boss type battle:
1) The Drider called Smeagchlol: In ages past a Yochlol displeased Lolth, the nature of this displeasure is lost to time but the punishment lives on. The Yochlol was fused with the Adamatine ore veins that surrounded one of lolths lost shrines in some far off region of the Underdark and the Yochlol was tasked with guarding the shrine and its precious treasure, the treasure consists of a single Diamond. The Yochlols form is now one of a drider like entity, robbed of its ability to shapechange it resembles a Drider with a metallic lower spiderl-ike body and the top half resembling the prior yochlol form. For stats combine a Iron Golem with a Yochlol, take on the Adamantine armour feature that turns critical hits into normal hits and you should have somehting pretty special.
2) The Slaves of Deep Duerra: In a large cavern where few dare to tread lies a pool of stagnant water. Within its depths lies the decayed remains of an elder brain. Surrounding this pool are smaller pools that are tended to by a handful of Illithids. Within these pools small tadpole like creatures swim and feed on each other. All of this is directed by a wizened Dwarf that sits upon a crystal throne, only roousing to feast on the tdpoles or issue commands to the Illithids which are under its thrall. This Dwarf is a Priestess of Deep Duerra and raises special weapons to serve in the Duergars enternal war against the Illithids and various other enemies. The Priestess' power is focused through a special diamond and she can use it to call upon two special weapons to defend her in the event of a thrall uprising or some other intruders, the weapons in question are two Neolithids that slumber under a lake that feeds the pools in the area.
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* Need a character idea? Search for "Rob76's Unused" in the Story and Lore section.
So we've finished the Beyond Icespire Peak trilogy and I'm home-brewing an add-on adventure to take the party to L20. I'm not familiar with the various monsters, so I'm asking if you have any suggestions for the final fight. Some background:
The party consists of an Evocation Wizard, a Valor Bard and a Champion Fighter (all straight up, no MCs).
The background of the adventure is that during the trilogy, the characters made a serious mistake and accidentally killed 7 innocents. They're now on a quest for seven special diamonds that will allow them to resurrect those dead and undo their mistake (I know that's not how the spell works, but we agreed on it to provide the plot motivation). We'll be working through several modified Candlekeep Mysteries that will get then researching the locations of the diamonds and going out to get them, plus a few homebrew quests.
My thought was that during the last quest, which is to travel into the Underdark, they'll accidentally awaken a great beast of some kind, which will go on a rampage and they have to, once again, clear up their mess, hopefully before it kills too many people.
Any suggestions as to what I could use? We can be fairly flexible with the lore. Or I could even have them awaken it earlier in the campaign but it's only at the end that the news of it catches up with them etc, so lore isn't a massive issue.
Thoughts?
If you're not willing or able to to discuss in good faith, then don't be surprised if I don't respond, there are better things in life for me to do than humour you. This signature is that response.
I mean, the classic 'great beast that goes on a rampage' is the Tarrasque, though it's a pretty poorly designed boss so you might want something homebrew. There aren't a lot of critters that are a decent fight for even three level 20 PCs, mostly greatwyrm dragons and divine avatars, and even fewer that are particularly inclined towards mindless rampages, though you could have something gone mad.
Homebrew is definitely the way to go. While many Monsters above CR 20 have creative powers, they still need Buffs and not Nerfs. Additional hit points (sometimes a lot), more Resistances (including an extra Saving Throw Proficiency) and Immunities (don't forget Conditions), and here and there a buff to damage die (meaning 1 or even 2 extra die and/or increasing the die size (d8 to d10 by example). One thing I do also is look at Monsters of both the same type as well as close to the same CR to get a feeling there. And obviously Action Economy, which can include Speed and type of movement. For Legendary Actions and Resistances, I generally give 4 to Monsters of CR 22 to CR 26 or 27, and 5 for CR's above that.
I'd offer more information about the Campaign specifically but I'm not familiar with it, sorry.
Edit: Pantagruel is absolutely correct about the Tarrasque in that usage, and it is also a prime example of needing the Buffs I mentioned. The thing should basically be immune to every condition there is, but definitely not Prone, if only for the possibility of it falling down on a row of buildings, crushing everything beneath it. A prior edition suggested the Tarrasque as having a sort of aura like a gravity well that prevented creatures from flying too high off the ground around it, but others may feel this is a cheat or something. I do know that there are a lot of people that have homebrewed their own Tarrasque in the face of the 5e pale imitation.
I like the idea of a modified version of juiblex oozing itself out of the underdark.
The mindless rampage isn't set in stone... that's just a thought. The bit I want is that they cause a disaster and end up fighting a showdown type monster/enemy.
As far as the So far, I've got:
A homebrew quest to find a rare tome so that they can enter Candlekeep - involving vampires.
The Inner Alchemy - modified to be a book that contains the ritual.
An unwritten homebrew involving Dwarves.
The Canopic Being - The missing sage knows the location of the first diamond, the party has to track her down and in the process finds the diamond after defeating the Mummy Lord that is behind her disappearance.
An unwritten homebrew, involving a Dragon Cult that we met previously in the trilogy.
The Scrivener's Tale - pretty much as written, except the book allegedly holds the location of a second diamond (but doesn't, and they're cursed).
Another unwritten homebrew, in which they find the third diamond.
Alkazar's Appendix - The Nether Scroll holds the locations of the final four diamonds, involves an Adult Blue Dracolich.
Xanthoria - they're infected while looking for a diamond in a village. A Lichen Lich took the diamond and infected the villagers, with each part being a quest.
A homebrew where they have to raid a dragon's lair for a diamond (will be in the form of two quests).
A homebrew where they have to enter the Underdark to retrieve a diamond (will be in the form of two quests). I'm unsure as to whether to go with Drow, Enchaphalids, or just go for a creature feature.
The final quest is that they've someone awakened or triggered a serious threat. Hopefully I can work it so that defeating the threat gives a diamond - it's carrying or powered by the diamond or something - but otherwise there will be another homebrew quest before this so they can get the diamond, possibly involving the Mummy Lord from Canopic Being returning, depending on how they resolved the quest.
If you're not willing or able to to discuss in good faith, then don't be surprised if I don't respond, there are better things in life for me to do than humour you. This signature is that response.
If your PCs have no built their characters erratically, there is no single monster in any of the books that provides them more than about 3 turns of combat. They are almost all based on a single large (but not very large) AoE attack (or comparable) and then getting punched to death quite fast after that.
The Lichen Lich is a good example of this. Fire Storm is very weak (38 damage on a failed save) and then it basically has nothing to threaten the PCs with. It has an absolutely wretched 225 hit points. An optimised level 11 party of 4 are going to bring it down in 2 turns with ease. I really don't understand how it gets its CR18, I'd put it at about CR11.
So homebrew something. Homebrewing monsters is really good fun, and the more you do it, the better you'll get.
If not then I suggest one of the mythic dragons from Fizbans are probably your best bet.
I"m liking this creature:
https://www.dndbeyond.com/monsters/1804891-fiend-of-the-mirror-world
Reflective surfaces are everywhere and you can give eerie hints that the party is being watched through them.
"Sooner or later, your Players are going to smash your railroad into a sandbox."
-Vedexent
"real life is a super high CR."
-OboeLauren
"............anybody got any potatoes? We could drop a potato in each hole an' see which ones get viciously mauled by horrible monsters?"
-Ilyara Thundertale
How is being dropped in two turns by an optimized level 11 party unexpectedly weak for a CR 18? AC 20/225 hp is a hair on the low side, but not dramatic, and even if never uses fire storm it has pretty substantial damage output (spamming poisonous touch is 122 dpr).
Two words: White Greatwyrm.
Panda-wat (I hate my username) is somehow convinced that he is objectively right about everything D&D related even though he obviously is not. Considering that, he'd probably make a great D&D youtuber.
"If I die, I can live with that." ~Luke Hart, the DM lair
The issues I've found with the Great Wyrms is that they are listed as being more challenging than an Ancient Dragon but they are obviously statistically much weaker. People counter that by pointing out they effectively have more hit points (but still, less combat power) than an Ancient, due to their Mythic status. But that itself doesn't answer why, if the higher challenge rating is due to resiliency, why the players should gain twice the experience of a CR 27 encounter. It shows the Great Wyrm is supposed to be equivalent to a back to back encounter but somehow the monster itself doesn't earn the CR of either the first or second half or that encounter.
Honestly, two rounds for a single creature isn't that bad for a standard WotC monster, but that is still really sad. Most parties (and I checked with a Poll in the forums a while ago) prefer combats that last 4 to 5 rounds. Still. Boss monsters should always be legendary at a minimum, and standard WotC monsters clearly show they need a lot more hit points and better defenses, especially for "elite / lieutenant" Monsters, not to even speak of Bosses. Further these monsters deserve a lair that rewards their own maneuverability.
I've always wanted to use this badboy in a campaign, but haven't had the chance. It's number of attacks/AC/HP can be adjusted if they're too low for your group.
Or you could just put three or four of them in a keep or sewer and have them patrol around. Might be something your players think they can handle, but when two or three of them show up in might be a different story. Live in cold, dank caves or dungeon complexes.
Monstrous melee attack creature that specializes in paralyzing and inflicting the frightened status. Heals itself and tramples its prey before raining blows.
They can have nests of 3-4 cubs.
The Wampus
The core problem isn't actually with monsters, the core of the problem is with PCs. If two equal-CR monsters get into a brawl, the typical duration of that fight will be 4-5 rounds, which is abut right for a coinflip difficulty fight. If two equal level PCs get into a brawl, you're likely talking about 2 rounds.
I don't know what you mean by greatwyrms being weaker than ancient. For most dragons all of their attacks do more damage than the ancient counter part. There are a couple of the higher cr ancient dragons which do slightly more damage on their breath weapon but the greatwyrm gets a much larger targeting area and a higher save dc in return so it balances out. Its just a result of their basically being 3 greatwyrm stat blocks as opposed to 6+ ancient dragon stat blocks. Some of the ancient dragons have above typical damage on their breath.
Any way, the way you make monsters last longer if you want a single monster in dnd is damage gates. Basically things like the mythic trait that break the hit points up into sections so they they aren't all blown through at once. It can be different forms like Auriel, Mythic traits, Contingencies or polymorphs just something that caps player damage like phases in a boss fight. It functionally turns one monster into several monsters run consecutively.
We must be looking at totally different stat blocks in relation to the Great Wyrm and the Ancient Dragons. Ancient Dragons have more hit points (if you take away the Mythic aspect), their breath weapons definitely deal more damage, and their Bite / Claw / Claw attacks and damage are equal or close to. It is weird that in a number of cases the Wyrms deal force damage rather than the Elemental damage. Also, the force spears seem out of place for a Dragon, and weak. Honestly, there doesn't seem to have been much in the way of creative advancement especially, not to mention general combat effectiveness. If you like the Great Wyrms, obviously enjoy, but I'm not seeing how the math supports a more challenging monster (again, noting either the individual before and after incarnations of the Mythic, which are all that should matter seeing as how with Mythic it really just comes down to a back to back encounter).
Edit: Just double checked myself, and the Ancients aren't extraordinarily more powerful than the Great Wyrms (using the Red Dragon as reference) but they still are more powerful. For some reason I had thought the gap was bigger. Personally it's also weird to me that a CR 27 Legendary Monster doesn't have 4 Legendary Actions, but at least it gets 4 Legendary Resistances that also refresh on the Mythic transformation.
Yeah the red dragon is one of the high CR outliers, the ancient green, white, blue and black all have lower health than the chromatic greatwyrm. However, even looking at the red dragon v chromatic greatwyrm, the greatwyrm has a stronger bite, stronger claws, stronger legendary actions, higher speed, higher dcs, higher hit modifier and it's attack inflict conditions which the ancient red's doesn't. The great wyrm is a significantly harder fight unless you're talking about insanely high damaging pcs
A mythic is functionally two CR x monsters in a row, so we can analyze either phase as a single monster. We'll start with defense
So, defensively ancient and greatwyrm phase 1 are about equal, which means we need to compensate by offensive CR. Looking at dpr math, in three rounds we expect
The greatwyrm also has a lot more breath weapon range, but that's not officially part of CR math, so I have to agree they're overall about the same CR. My suspicion is that dragons in the monster manual are somewhat overpowered for their listed CR, and Fizban's actually used the official rules for CR.
@Pantagruel666 I actually should review some of the newer monsters under the proposed rating system of the DMG. I remember when it was first put out, people all thought it was BS because none of the MM entries matched up with their own CR, the general community assumption being that it was just a defensive gimmick to pretend there was an actual system when there wasn't one. That's what I remember anyway.
A couple of options come to mind for an underdark based boss type battle:
1) The Drider called Smeagchlol: In ages past a Yochlol displeased Lolth, the nature of this displeasure is lost to time but the punishment lives on. The Yochlol was fused with the Adamatine ore veins that surrounded one of lolths lost shrines in some far off region of the Underdark and the Yochlol was tasked with guarding the shrine and its precious treasure, the treasure consists of a single Diamond. The Yochlols form is now one of a drider like entity, robbed of its ability to shapechange it resembles a Drider with a metallic lower spiderl-ike body and the top half resembling the prior yochlol form. For stats combine a Iron Golem with a Yochlol, take on the Adamantine armour feature that turns critical hits into normal hits and you should have somehting pretty special.
2) The Slaves of Deep Duerra: In a large cavern where few dare to tread lies a pool of stagnant water. Within its depths lies the decayed remains of an elder brain. Surrounding this pool are smaller pools that are tended to by a handful of Illithids. Within these pools small tadpole like creatures swim and feed on each other. All of this is directed by a wizened Dwarf that sits upon a crystal throne, only roousing to feast on the tdpoles or issue commands to the Illithids which are under its thrall. This Dwarf is a Priestess of Deep Duerra and raises special weapons to serve in the Duergars enternal war against the Illithids and various other enemies. The Priestess' power is focused through a special diamond and she can use it to call upon two special weapons to defend her in the event of a thrall uprising or some other intruders, the weapons in question are two Neolithids that slumber under a lake that feeds the pools in the area.