Mimic

A mimic ally is not friendship, it is a working relationship with a trained predator. The framing that holds is a thieves' guild that uses one as a vault guard, a wizard who has bound one to a specific room, or a tavern owner whose mimic eats anyone who tries the till after hours. The party hires the mimic's owner. The mimic has Intelligence 5, which is enough to recognize one or two handlers and tell them apart from food, and not much else.

Use it for a stationary job. The mimic shape-shifts to whatever object the room needs, sits there for hours or days, and grabs whoever touches the wrong thing. Adhesive at escape DC 13 with Disadvantage means the target is stuck for the rest of the scene unless an ally with a free hand spends an action helping. The party's job, if they are running the heist, is to make sure the wrong person reaches in first and the right people stay out of arm's reach. Pseudopod has a 5-foot reach and Bite has 5-foot reach, so the working radius is small and the players can plan around it.

In a fight, the mimic stays where it was placed. It does not chase or flank or do anything resembling tactics. It eats whatever comes within reach. If the party needs the mimic to cross a room, the handler has to direct it physically, which means walking it across like a shop awning that has decided to follow.

When the job ends, the handler feeds the mimic something fresh, which the players should be allowed to not look directly at, and packs it back into the cart. The party owes the handler in coin and in not asking questions about where the next meal is coming from.

Describe the mimic's handler whispering to it like a falconer. Players remember a handler with a name long after the mimic itself becomes a footnote.

A mimic is a one-trick monster, but the trick is the entire encounter. The party has to touch it to find out, and once they touch it the Adhesive trait says they are stuck. The fight is not the chest. The fight is the panic that follows the moment a hand cannot come off the lid. Place the mimic where greed routes the party: the chest at the back of the kobold den, the door at the end of the corridor, the bookshelf in the dead wizard's study. The mimic can shape-shift to a Medium or Small object, so the menu is wide.

When the trap springs, the toucher is Grappled at escape DC 13 with Disadvantage on the check. That player is not getting free this round. Open with Bite at +5 with Advantage from the Grapple, for 12 piercing plus 4 acid, average 16 a turn on a target that cannot move. On the second round, send a Pseudopod at the rescuer who closed to 5 feet, since Pseudopod's Grapple turns one stuck PC into two stuck PCs. The mimic cannot Multiattack, so the choice each turn is which target to commit Bite to and whether to grab a new victim with Pseudopod.

The mimic's defenses are thinner than its offense. AC 12, 58 HP, immunity to Acid and the Prone condition, Darkvision 60 ft., Stealth +5. There is no Multiattack, no recharge ability, no escape. The party will kill it in three rounds once they stop trying to pull free and start hitting it from range. Reward the wizard who casts Fire Bolt over the rogue who keeps trying to yank their hand back. Adhesive only triggers in object form, so a mimic that drops to true blob form to chase a fleeing PC has surrendered the grappling advantage that made it scary.

A mimic does not retreat. It is a hungry predator with a Wisdom of 13 and an instinct that says the prey it has caught is the prey it eats. Once Bloodied, it Shape-Shifts back to true form to chase, which is a tactical mistake the party should be allowed to exploit.

Roll the mimic's first Bite attack openly and announce the +5 Advantage from the Grapple before the dice land. The math is the horror.