Is there any monster with statblock changes during an encounter?

Question

So I've been DMing D&D 5e for quite some time. My table enjoys long, challenging encounters. However many monsters end up repeating the same actions after a few rounds. I usually circumvent this issue by making my BBEG a spellcaster, or adding external elements to the bad guy's lair (such as lair actions, or some sort of narrative countdown). This way I make sure that every round is unique and that my players are engaged during the fight.

However I have the feeling I could do better if the bad guys could change over time, making them more lethal or simply changing their mechanics as the encounter moves to the end, adding some dramatic and epic scenes. Let's say a black dragon that, when dropped below 50 HP, loses their flying speed but instead they have some acid area-of-effect centered on them every time they use their breath weapon or are attacked.

Are there any monster statblocks that include something like this? What would be the best way to balance such an encounter?

Answer 1

Auril, from Icewind Dale: Rime of the Frostmaiden

Auril has three entirely different stat blocks, for three entirely different forms, each of which gets released when you defeat her previous form.

Mythic Odyssey of Theros Mythic Monsters

Mythic monsters have an optional ability to transform themselves to make for more exciting battles.

[...] Mythic traits are optional; they don't need to be used during combat with these monsters. If you so choose, you may simply ignore a monster's mythic trait and mythic actions. If you wish to increase a battle's stakes, though, using a monster's mythic trait results in some mid-battle twist that changes the way the monster behaves, restores its resources, or provides it with new actions to use. As a result, the battle becomes deadlier and rages on for longer than most combat encounters. [...]

The exact transformation effects differ from monster to monster. They all are of high challenge rating of 17 to 26, and if you use the mythic trait, they count as two monsters of said challenge rating for XP purposes.

Additional mythic monsters can be found in Van Richten's Guide to Ravenloft (the Dullahan, CR 10), and in Fizban's Treasury of Dragons (various Dragons – the Ancient Dragon Turtle, metallic and chromatic Greatwyrms, and Aspects of Bahamut and Tiamat, ranging from CR 24 to 30). [Credit for these goes to this answer from Thomas Markov.]

Answer 2

Whilst Groody answered the question asked, I have had some similar home-brewed monsters in my campaigns which may offer insight into problems you might have if you introduce such mechanics to your encounters, such as the dragon you gave as an example:

The first of those was a barbarian-esque warrior that on hitting 100 hp, for a party of 3 level 18 characters, that got knocked down to that by the fighter and then got instantly power word killed. I'd considered it could be burst at this point so gave it resistances but hadn't considered instant kills, so make sure to be careful for things like that.

The next one actually went relatively well, it was a Giant Crystalline Lobster that on hitting 100 hp, for a party of 5 level 5 characters, that lowered its' AC and Dex/Con by -2 and -1 respectively. But in the process released the magic within it through the cracks that had formed to allow it to cast with legendary actions, and started playing more defensively and trying to get away whilst still defending itself. This changed the feel of the fight, and actively made it feel like the party got a foot-hold over a threatening monster. I ended up using this style as my go to for adding mechanics like this, essentially keeping the monster itself the same occasionally with a debuff and opening up lair or legendary actions.

The final relevant, different, one was similar to Auril where new phases happened on death (Specifically not on 0 hp, due to the first experience), and had 4 phases which was just far too many and ended up being a 12 turn encounter. This also ended up just feeling like a gimmick fight due to the party not knowing how many phases would happen with them presuming they had to do something specific to win. So if you choose to use something similar to this where it's obvious that they die and respawn make sure to either keep it short, maybe 1 respawn, or that the party knows how long it will be before they start the encounter.

Answer 3

There are some examples in Fizban's Treasury of Dragons.

There are several monsters in Fizban's Treasury of Dragons, such as the Aspect of Tiamat (FToD, pg. 166), the Aspect of Bahamut (FToD, pg. 165), Greatwyrms (FToD, pg. 201), Ancient Dragon Turtle (FToD, pg. 191), which have a trait that activates under particular conditions, allowing to regain some of their abilities and acquiring new ones.

The Aspect of Tiamat's one is provided below:

Chromatic Wrath (Recharges after a Short or Long Rest). If the aspect would be reduced to 0 hit points, her current hit point total instead resets to 500 hit points, she recharges her Chromatic Flames, and she regains any expended uses of Legendary Resistance. Additionally, the aspect can now use the options in the "Mythic Actions" section for 1 hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating the aspect of Tiamat after her Chromatic Wrath activates.

The new options are

the Mythic actions, which are described as

Mythic actions. If the aspect’s Chromatic Wrath trait has activated in the last hour, she can use the options below as legendary actions.

The two mythic actions are Hurl Through Avernus and Chromatic Flare.

Answer 4

Rethinking Solo Monster Design

Your issue with strong monsters becoming boring and predicable over the course of a long encounter is a common issue with 5th Edition D&D (and to some degree all editions). There are several reasons for it, but most boil down to 1 big monster vs 4-5 adventurers being either A. an easy victory for the party due to action economy or B. a long drawn out slog because the DM buffed HP and AC to compensate for A.

Your instincts to add external elements such as lair actions or minions is a good one. A changing battlefield is a great way to keep long encounters interesting. But I would like to direct you to what I think is the single best resource on this topic:

Matt Colville's "Action Oriented Monsters" Video

This masterclass of a video focusing on addressing this exact issue by changing the way we think about running single powerful monsters. A quote from the video:

The heroes pound on the monster, because there are so many heroes and only one monster, the heroes do a ton of damage and then one of two things happens. The monster dies without ever getting to do anything (the definition of anticlimax) or the monster acts and if it was tough enough to survive the entire party pounding on it then it's really nasty and when it attacks it will probably kill a character. That is not fun.

So that's the problem, a challenging solo monster or boss can't be about "toughness" because the way the game is designed, any monster tough enough to survive the heroes pounding on it, is nasty enough to kill them.

The video focuses on building better solo monsters to address this. There are a few main themes:

• High level monster / PCs are complicated to run.
  • Remove the complexity by focusing on combat stats.
  • Ignore skills and non-combat abilities for monsters.
  • Simplify spells but just picking one or two to cast and not tracking spell slots.
• Focus on Actions.
  • Actions the solo monster or boss will take on their turn
  • Bonus Actions they can use to influence the battle
  • Give your monsters interesting Reactions to specific triggers.
  • Adding a custom version of Legendary Actions called Villain Actions.
    • Keep them on theme for the monster
    • A unique villian action per round of combat
• Have your monsters actions make the fight more dynamic
  • Design your Villian, Legendary and Lair actions in a way that makes the fight more interest
  • Promote movement, changing the environment and forcing players to think on their feet.

Overall this video is a brilliant resource for building better solo and boss monsters. I highly recommend watching the whole thing, probably more than once.

Multiple HP pools

An alternative homebrew rule that's more in line with your original question, is to use multiple hit point pools for your solo and boss monsters. This can easily be used with the action oriented monsters I mentioned above.

Instead of buffing your monster to incredibly high numbers for AC and HP in order to keep them alive. Give them 2-3 HP pools and work through each sequentially. Each pool relates to a phase of the fight and once depleted there should be a noticeable narrative and mechanical effect on the fight.

Many DMs, online and otherwise have their own take on the HP pool method, so I encourage you to do your own research on this topic. If you want to keep it simple, just provide a simple buff or change of tactics after each stage. If you want a bigger change you can completely replace their statblock with a different one. Effectively you are running separate encounters back-to-back so you can make the different phases of the monsters statblock as different or similar as you wish.

Answer 5

Possession

Monsters that possess targets, such as the ghost and intellect devourer (MM, p. 191), change their stats during combat.

For instance, the description of the Ghost's Possession action states:

Possession (Recharge 6). One humanoid that the ghost can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.

Answer 6

I've seen 3rd party products that include such changes, like "if the orc leader dies, the others will rally in anger, gaining 10 temporary HP and an immediate opportunity attack" or something like that. The wrote them for many creatures or groups - you can create something similar if you have the time and inclination, but you can also buy it.

Another piece of advice I liked - and I wish I'd kept the link - is to make every encounter unique. Add personality to the enemies, add an interesting field of combat with traps, bridges, and elevation, add 3rd parties that wait to attack whoever wins, or help out if you're a (foolishly) kind GM, etc. It is more work for you, but at least you won't hear "Oh, it's three owl-bears this time" when one of them turns out to be an illusionist in disguise or they run right past the party because of the three undead owl-bears right behind them. ;)