Adhesive. The mimic adheres to anything that touches it. A Gargantuan or smaller creature adhered to the mimic is also grappled by it (escape DC 18). Ability checks made to escape this grapple have disadvantage.

Amorphous. The mimic can move through a space as narrow as 1 foot wide without squeezing.

Corrosive Form. A creature that touches the mimic or hits it with a melee attack while within 5 feet of it takes 4 (1d8) acid damage. Any nonmagical weapon made of metal or wood that hits the mimic corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed.

Nonmagical ammunition made of metal or wood that hits the mimic is destroyed after dealing damage.

The mimic can eat through 2-inch-thick, nonmagical wood or metal in 1 round.

False Appearance. While the mimic's eyes and mouth remain closed, it is indistinguishable from an ordinary storm cloud.

Flammable Gas. If the mimic takes fire damage, it descends 60 ft. and is unable to ascend any higher than its current altitude until it finishes a long rest.

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

Innate Spellcasting. The mimic’s spellcasting ability is Constitution (spell save DC 17). It can innately cast the following spells, requiring no material components:

At will: gust of wind, thunderclap (as a 5th-level caster, 2d6 thunder damage on a failed save)

3/day each: lightning bolt

Troposphere Traverser. A creature cannot attack the mimic unless it is able to fly, has an attack range greater than 100 feet, or the mimic has descended into the creature's attack range.

Pseudopod. Melee Weapon Attack: +10 to hit, reach 20 ft., one target. Hit: 32 (4d12 + 6) bludgeoning damage. If the mimic is in object form, the target is subjected to its Adhesive trait.

Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 32 (4d12 + 6) piercing damage plus 26 (4d12) acid damage. If the target is a Large or smaller creature, it must succeed on a DC 18 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 21 (6d6) acid damage at the start of each of the mimic's turns.

If the mimic takes 20 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. If the mimic dies, a swallowed creature is no longer restrained.  Due to the mimic's amorphous form, no special movement is required to escape its corpse; however, its corpse is difficult terrain for any creature without a swimming speed.

Acid Rain. As a bonus action, the mimic secretes acid which falls like rain. Each creature and object under the mimic takes 1d8 acid damage.  Any nonmagical armor a creature is wearing corrodes. The armor takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5 or reduces the creature's AC to a value equal to its Unarmored AC, the armor is destroyed.