CoS Greater Mimic

Towering Resistance (2/Day). If the mimic fails a saving throw, it can choose to succeed instead. When it does so, one creature inside of it can use its reaction to leave the mimic's body and move into an unoccupied space within 5 feet of it.

Complex Interior. A creature can occupy the same space as the mimic. Creatures inside the mimic's body must spend up to 80 feet of movement to reach an exit (exact distance determined by the GM). This movement can be spread across multiple turns.

False Appearance. If the mimic is motionless at the start of combat, it has advantage on its initiative roll. Moreover, if a creature hasn't observed the mimic move or act, that creature must succeed on a DC 25 Intelligence (Investigation) check to discern that it is more than an ordinary structure.

Slow Shifter. With 24 hours of time, the mimic can change its exterior appearance or form, including door or window-like exits.

Damage Threshold. The mimic has a damage threshold of 20 against damage from outside its body and 10 against damage from creatures inside it.

Colossal Form. The mimic can occupy a space larger than the standard 20x20 feet. Further, it can't be forcibly moved.

Multiattack. The mimic makes three Internal Bite attacks.

Internal Bite. Melee Attack Roll: +13, reach 0 ft. Hit: 15 (2d6 + 8) Bludgeoning or Piercing damage (mimic's choice)

Structure Slam. Constitution Saving Throw: DC 21, Each creature in a 5-foot radius centered on a point outside the mimic's body within 20 feet of it. Failure: 47 (6d12 + 8) Bludgeoning damage. Success: Half damage.

Spellcasting. The mimic can cast Prestidigitation or Silent Image at will, requiring no spell components and using Constitution as the spellcasting ability (spell save DC 20).

Long Tongue. Dexterity Saving Throw: DC 21, one Large or smaller creature the mimic can see within 50 feet it. Failure: The target is pulled inside the mimic's body.

Legendary Action Uses: 3. Immediately after another creature’s turn, the mimic can expend a use to take one of the following actions. The mimic regains all expended uses at the start of each of its turns.

Internal Bite. The mimic makes one internal bite attack.

Interior Shift. Strength or Dexterity Saving Throw (Creature's choice): DC 20, each creature of the mimic's choice inside its body. Failure: The target's distance to an exit is increased by 20 feet (up to a maximum of 80 feet), and it is grappled (escape DC 21). Success: The target's distance to an exit is increased by only 10 feet. Faillure or Success: The mimic can't take this action again until the start of its next turn.

Violent Shaking. Constitution Saving Throw: DC 20, each creature of the mimic's choice inside its body. Failure: The target is knocked prone. Failure or Success: A creature climbing on the mimic falls and lands in an unoccupied space within 5 feet of it. Further, the mimic can't take this action again until the start of its next turn.