Shapechanger. The mimic can use its action to polymorph into a dungeon 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.  The mimic can also make alterations to its dungeon form to create or dismantle features such as puzzles, traps, and walls.  The mimic uses it ability to shapechange to form eyes along its interior, allowing it to keep track of the location and activities of creatures inside it.

Damage Threshold (20):  An attack must inflict at least 20 damage, after applying resistances and immunities, to harm the mimic.  Otherwise, the mimic takes no damage.

Adhesive (Object Form Only). The mimic can adhere to anything that touches it, and chooses which targets it adheres to and which it doesn't. A Gargantuan or smaller creature adhered to the mimic is also grappled by it (escape DC 26). Ability checks made to escape this grapple have disadvantage.

False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from a typical adventuring dungeon.

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

Innate Spellcasting. The mimic's innate spellcasting ability is Intelligence (spell save DC 21, +13 to hit with spell attacks). The mimic can innately cast any spell which replicates the effect of a trap or puzzle at the discretion of the DM.  It also can cast the following spells, requiring no components:

At will: detect magic, detect thoughts, identify.

Risk and Reward: A creature reduced to 0 hit points while challenging the mimic's dungeon dies and is devoured by the mimic.  A creature that fails to follow the dungeon rules and receives 3 warnings is attacked by the mimic.  Successfully completing dungeon challenges awards higher end treasure appropriate for that creature's level.

Scaling Dungeon. The mimic can use its spell DC as the DC for any checks a creature makes in order to succeed against the obstacles it generates.  The mimic can scale its spell DC to accommodate the ability level of an adventuring party challenging it based on the average level of the party.

Pseudopod. Melee Weapon Attack: +18 to hit, reach 20 ft., one target. Hit: 36 (4d12 + 10) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.  If the target is a creature, it must succeed on a DC 26 Strength saving throw.  On a failed save, the target is pulled to a space within 5 feet of one of the mimic's walls, which forms a set of jaws.

Bite. Melee Weapon Attack: +18 to hit, reach 5 ft., one target. Hit: 36 (4d12 + 10) piercing damage plus 26 (4d12) acid damage. If the target is a Huge or smaller creature, it must succeed on a DC 26 Strength or Dexterity saving throw (target's choice) or be swallowed by the mimic. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside the mimic, and it takes 42 (12d6) acid damage at the start of each of the mimic's turns.

If the mimic takes 40 damage or more on a single turn from a creature inside it, the mimic must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the mimic, outside of the mimic's dungeon. If the mimic dies, a swallowed creature is no longer restrained by it and can escape from the corpse, treating the amorphous mimic's corpse as difficult terrain caused by liquid; a creature with a swimming speed can move normally.

Collapse (Recharge 6). The mimic collapses a room inside itself.  Any creature in that room must succeed on a DC 26 Strength or Constitution saving thrown (target's choice), taking 208 (32d12) bludgeoning damage on a failed saving throw, or half as much damage on a success.  Regardless of the saving throw's result, the creature is swallowed by the mimic, as outlined above.

Set Challenge. When a creature first enters a new room in the mimic's dungeon, the mimic creates one of the following challenges:

• Combat Encounter.  The mimic generates 1d4 simulacrum which must be defeated in order to advance further into the dungeon.  Simulacrum are outlined in the Dungeon Master's Challenges section under "Combat Challenges" below.
• Puzzle:  The mimic sets a puzzle which must be solved in order to advance further into the dungeon.  A puzzle may incorporate a combat encounter or a trap, but should not incorporate both; but is ultimately at the discretion of the DM.
• Trap:  The mimic sets a trap which must be dealt with in order to advance further into the dungeon.  Traps are outlined in the Dungeon Master's Challenges section under "Traps" below.