Everlasting Evolution. Upon death, quickly revives and rapidly adapts to resist whatever damage type killed it, except Necrosis and Radiant, up to the amount of centuries it has lived.

Crushed (due to size difference) - it's size increases to Large if tiny and Gargantuan if Large (ALWAYS starts as tiny)

Bludgeoning/Force - Develops Air Sacs around it's body that bounce bludgeoning attacks away, but are susceptible to being pierced.

Piercing/Slashing - Develops a hard shell that is difficult to be pierced through but is susceptible to being crushed.

Fire - It's body temperature reduces to such a temperature that fire instantly goes out but is more susceptible to the cold.

Cold - It gains the ability to ignite itself suddenly but will be ignited in the process.

Psychic - It starts gaining the intelligence and wisdom of anyone it eats.

Thunder -  It loses the ability to hear, also making it immune to spells that require the caster to be heard.

Acid - Starts secreting a powerful base that neutralizes the acids as they come into contact with them, the heat of the reaction still hurts it.

Lightning - A couple of Antennae grow from it's head that draw lightning towards them, the lightning is sent through specialized organs and give it a single use of lightning breath.

Poison - While depending on the speed of the poison they might still work, it gains the ability to use a bonus action each turn to instantly synthesize an antidote to the poison and then can never be affected by that poison again. 

Squirm. When the Adapt Horror moves it deals damage based on it's size

Tiny - 0d4 

Large - 2d8

Gargantuan - 8d12

Squishable. Can be one-shot by being stepped on if size is 2 or more sizes smaller than the other creature.

Lightning Breath. The Adapt Horror releases absorbed lightning in a 45-­foot line that is 5 feet wide. Each creature in that line must make a DC 19 Dexterity saving throw, taking damage equal to the lightning damage of the attack that granted it a charge on a failed save, or half as much damage on a successful one.

Poison Adaption - If it has died to poison once, it can then use a bonus action each turn to develop an antidote that instantly cures a single poison that can never effect it again.

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