Great Mimic (5e Creature)
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Medium monstrosity (shapechanger), any chaotic alignment
Shapechanger. The mimic can use its action to polymorph into an object, one humanoid it can see, or back into its true, amorphous form. Its statistics are the same in object and true forms, but it gains any racial traits, speed, and known cantrips (if any) of any humanoid it copies. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
Adhesive (Object Form Only). The mimic adheres to anything that touches it. A Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 13). Ability checks made to escape this grapple have disadvantage.
False Appearance (Object Form Only). While the mimic remains motionless in object form, it is indistinguishable from an ordinary object. When interacting with the mimic in humanoid form while it is not hostile, a person may make a DC 20 Wisdom (Perception) check to identify the humanoid as an imposter. While hostile, the mimic is identifiable.
Multiattack. The mimic makes three attacks, two with its pseudopods and one with its bite.
Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.
Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 18 (2d12 + 5) piercing damage plus 6 (1d12) acid damage. The target must succeed on a DC 15 Constitution saving throw or become poisoned for 1 minute. On a result of 5 or lower, the target is also stunned for the same duration. A creature can repeat its saving throw at the end of each of its turns, ending the poison on a success. If the target is also stunned, a success removes the stunned condition, but another success is required to end the poison.
Mimics are opportunistic hunters, who take the form of objects to lure in prey, then attack with pseudopods and deadly bites. A great mimic is a much more powerful form of mimic, with higher intelligence and the ability to manifest the form of humanoids. They may be created from ordinary captured mimics by a ritual, or may simply be a free-spirited mimic who devoted itself to the pursuit of knowledge and power rather than food. Great mimics' impersonations of humanoids are limited. They can near-identically change into people on sight, but almost always get the details wrong by memory, and come out closer to their true form. They are usually bound in service to spell casters who empowered them, used as spies and assassins. Watching what might've been an ally sprout long pseudopods and a near-dislocated, long-tongued mouth and attack is a truly frightening ordeal. However, mimics that transcended on their own accords, however extremely rare, follow their own paths. By mastering their bodies' manipulations, great mimics' bites produce more potent corrosion, and can infect their victims with a deadly neurotoxin that leaves them helpless.