Chamber Mimic

Partial Shapechanger. The chamber mimic can use its action to polumorph parts of it's body into parts of the chamber it inhabits, objects inside the chamber, 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.

Willingly Adhesive (Chamber Form Only). The chamber mimic can make parts of its body adhere to anything that touches it. If a Huge or smaller creature adhered to the chamber mimic is also grappled by it (escape DC 19). If a creature starts it's touching any surface of the mimic, it must make a DC 19 Strength save or be adhered to the chamber mimic. Ability checks made to escape this grapple have disadvantage.

Digestion. A creature that starts its turn affected by the Willingly Adhesive trait takes 9 (2d8) acid damage. 

False Appearance (Chamber Form Only). While the chamber mimic remains motionless, it is indistinguishable from an oridnary chamber including any obejcts found within said chamber. 

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

Multiattack. The chamber mimic makes three attacks. Two with it's pseudopod and one with it's bite.

Pseudopod. Melee Weapon Attack: +12 to hit, reach any creature within the chamber mimic, one target. Hit: 26 (4d8 + 6) bludgeoning damage. If the chamber mimic is in chamber form, the target is subjected to its Willingly Adhesive trait.

Bite. Melee Weapon Attack: +12 to hit, reach any creature withing the chamber mimic, one target. Hit: 26 (4d8 + 6) piercing damage plus 20 (4d8) acid damage.