Arcane Mimic

Shapechanger. The mimic can use its action to polymorph into a creature, object, or back into its true, amorphous form. Its statistics (except for Intelligence and Wisdom) change to match the creature's. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

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

Spellcasting: The Arcane Mimic is an 18th level spellcaster, and always has the following spells prepared. It can cast all these spells without material components and uses Wisdom as its spellcasting modifier (spell save DC 18):

[At will]: Arms of Hadar, Color Spray, Disguise Self, Web

[2/day]: Counterspell, Hunger of Hadar, Polymorph

[1/day]: Immolation, Synaptic Static

Pseudopod (True Form Only). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage. 

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

Bash (Object Form Only). Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 7 (1d8 + 3) bludgeoning damage. 

Terrorise: The Arcane Mimic exposes the essence of the Shadowfell within. Each creature of the Arcane Mimics choice must make a DC 18 WIS save or take 17 (3d10) psychic damage and be stunned on a failed save or half damage and not be stunned. 

Teleport: The Arcane Mimic teleports up to 30 ft away from its current position.