Boss Mimic

Shapechanger. The mimic can use its action to polymorph into an object 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.

Super Adhesive. The mimic adheres to anything that touches it. A Huge or smaller creature adhered to the mimic is also grappled by it. It takes a DC 17 Strength save. Ability checks made to escape this grapple have disadvantage. If the mimic is hit with a melee weapon, the mimic may grab the weapon as a reaction. It takes a DC 17 Strength check to remove the item. If the mimic is holding a weapon, it can use the Weapon Hurl or Swallow Object abilities. 

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.

Devouring. The mimic heals hit points equal to half of any acid damage that it deals to other creatures. 

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

Pseudopod. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 10 (1d10 + 5) bludgeoning damage. The target is subjected to the mimic's Adhesive trait.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 11 (1d12 + 5) piercing damage plus 13 (2d8 + 4) acid damage. If the target creature is grappled, and the attack hits, the creature will be swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the mimic, and it takes 22 (4d8 + 4) acid damage at the start of each of the mimic's turns. The mimic can have two creatures swallowed at a time. If a creature is killed while inside, the mimic will receive extra temporary hit points equal to half of the creature's total hit points. If the mimic dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 5 feet of movement, exiting prone.

Weapon Hurl. If the mimic has a weapon stuck to it, it can use a bonus action to hurl the weapon. The thrown weapon will have +5 to hit, or a +9 for weapons that have the thrown trait. The thrown weapon will use it's own damage, and use the mimic's Strength for the damage modifier.

Swallow Object. If the mimic has a non-magical, medium object or smaller stuck to it, it can use a bonus action to swallow the object. If the item is made of wooden, cloth, or other organic materials, the item will be destroyed at the start of the mimic's next turn, and the mimic will gain 1d4+2 health points. If the item is made of a more durable materials such as metal or stone, it will slowly corrode and be digested. For each turn that a metal weapon is in the mimic, roll 1d4 + 2; the weapon gains this number as a permanent penalty to damage rolls. If the penalty equals or exceeds the weapons damage die, the weapon is permanently destroyed.