Shapechanger. The mimic can use its action to polymorph into a chimney or fireplace 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.

Adhesive (Object Form Only). The mimic adheres to anything that touches it. A creature adhered to the mimic is also grappled by it (escape DC 14). Ability checks made to escape this grapple have disadvantage.

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

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

Innate Spellcasting. The mimic's innate spellcasting ability is Constitution (spell save DC 14, +6 to hit with spell attacks).  The mimic can innately cast the following spells, requiring no components:

At will: create bonfire, produce flame

3/day each: burning hands

1/day each:  fire shield (warm shield, 2d8 fire damage)

Pseudopod. Melee Weapon Attack: +6 to hit, reach 10ft., one target. Hit 13 (2d8 + 4) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bite. Melee Weapon Attack: +6 to hit, reach 5ft., one target. Hit 13 (2d8 + 4) piercing damage plus 9 (2d8) fire damage.