Shapechanger. The mimic can use its action to polymorph into a door or window shutter 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.

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

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

Innate Spellcasting. The mimic's innate spellcasting ability is Strength (spell save DC 13). The mimic can innately cast the following spells, requiring no material components:

1/day each: arcane lock (self only)

Key Weakpoint.  When a creature makes a melee weapon attack against the mimic, they can make an ability check to unlock the mimic's lock instead of a normal attack roll.  On a success, the attack hits and is a critical hit.

Living Lock. The mimic functions as a regular locked door with a DC of 13 to unlock or break its lock.

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.

Slam. Melee Weapon Attack: +5 to hit, reach 5 ft., one target not grappled by the mimic. Hit: 12 (2d8 + 3) bludgeoning damage, and the target must succeed on a DC 13 Strength or Dexterity saving throw (target's choice) or be pushed 5 feet away from the mimic.  If the saving throw fails by 5 or more, the target also falls prone.