Shapechanger. The mimic can use its action to polymorph into a coffin 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.

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.

Corpses. When first encountered, the mimic contains the corpses of 1d4 - 1 humanoids.  The mimic can hold the remains of up to three dead humanoids.  These remains have total cover against attacks and other effects outside the mimic.  If the mimic dies, the corpses within it can be pulled free. While it has at least one humanoid corpse in its body, the mimic can use a bonus action to do one of the following:

• The mimic digests one humanoid corpse in its body and instantly regains 9 (2d8) hit points. Nothing of the digested body remains. Any equipment on the corpse is expelled from the mimic in its space.
• The mimic animates one dead humanoid in its body, turning it into a zombie.  The zombie appears in an unoccupied space within 5 feet of the mimic and acts immediately after it in the initiative order.  The zombie acts as an ally of the mimic but isn't under its control.

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary coffin.

Grappler. The mimic has advantage on attack rolls against any creature grappled by it.

Pseudopod. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) 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: 7 (1d8 + 3) piercing damage plus 4 (1d8) acid damage.

Premature Burial. The mimic makes one bite attack against a Medium 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 mimic, and it takes 7 (2d6) acid damage at the start of each of the mimic's turns.  A creature that dies while swallowed can be used with the mimic's Corpses trait.

If the mimic takes 10 damage or more on a single turn from a creature inside it, the mimic must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, including any corpses it contains, which fall prone in a space within 5 feet of the mimic. If the mimic dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 5 feet of movement.

Undertake. The mimic grabs one unsecured dead humanoid within 5 feet of it and stuffs the corpse into itself, along with any equipment the corpse is wearing or carrying. The remains can be used with the mimic's Corpses trait.