False Appearance (Hoard Form Only). If the mimic is motionless at the start of combat, it has advantage on its initiative roll. Moreover, if a creature hasn’t observed the mimic move or act, that creature must succeed on a DC 18 Intelligence (Investigation) check to discern that the mimic is animate.

Legendary Resistance (3/Day). If the ancient mimic fails a saving throw, it can choose to succeed instead.

Magic Resistance. The ancient mimic has advantage on saving throws against spells and other magical effects.

Reflective Carapace. Any time the ancient mimic is targeted by a magic missile spell, a line spell, or a spell that requires a ranged attack roll, roll a d6. On a 1 to 5, the ancient mimic is unaffected. On a 6, the ancient mimic is unaffected, and the effect is reflected back at the caster as though it originated from the ancient mimic, turning the caster into the target.

Siege Monster. The ancient mimic deals double damage to objects and structures.

Multiattack. The mimic makes one Bite attack and two Pseudopod attacks.

Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one creature. Hit: 16 (2d10 + 5) piercing damage plus 7 (2d6) acid damage.

Pseudopod. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage, and the mimic adheres to the target. A creature adhered to the mimic is also grappled by it (escape DC 16). Until this grapple ends, the target is restrained. Ability checks made to escape this grapple have disadvantage.

Caustic Mist (Recharge 5–6). The mimic sprays a fine mist of acid in a 30-foot cone. Each creature in that area must make a DC 14 Dexterity saving throw. On a failed save, the creature takes 27 (6d8) acid damage and is blinded until the end of its next turn. On a successful save, the creature takes half as much damage and isn’t blinded.

Shapechanger. The mimic transforms into a hoard or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn’t transformed. It reverts to its true form if it dies. 

Swallow. The ancient mimic makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the ancient mimic, and it takes 56 (16d6) acid damage at the start of each of the ancient mimic’s turns.

If the ancient mimic takes 60 damage or more on a single turn from a creature inside it, the ancient mimic must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the ancient mimic. If the ancient mimic dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 30 feet of movement, exiting prone.

The ancient mimic can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. The ancient mimic regains spent legendary actions at the start of its turn.

Attack. The ancient mimic makes two bite attacks or pseudopod attacks.

Move. The ancient mimic moves up to half its speed.

Chomp (Costs 2 Actions). The ancient mimic makes one bite attack or uses its Swallow.