Bonemantle

False Appearance. While staying still the bonemantle will look identical to a skeleton.

Tunneler. The bonemantle burrows through earth at half its burrow speed.

Bioluminescence. The bonemantle emits a bright light up to 80 ft away.

Dissolve. The bonemantle deals 12d12 + 144 to a creature it has swallowed and if the targets health is reduced to 0, it spits out its skeleton, and gains a bonus 4 points to its armor class.

Multiattack. The bonemantle can make one envelope/dissolve attack and two pincher attacks or three acid rays.

Envelope. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 3d10 + 5 bludgeoning damage and the target must make a DC 17 strength save, on a fail, the target becomes swallowed, while swallowed the target becomes restrained and blind. The target can escape the bonemantle on a success of a DC 15 strength save.

Pincher. Melee Weapon Attack: +12 to hit, reach 10 ft., 1 target. Hit: 5d12 + 7 bludgeoning damage. 

Acid Ray. RangedWeapon Attack: +8 to hit, range 10/120 ft., 1 target. Hit: 5d10 + 5 acid damage.

The bonemantle takes 4 mythic actions at the beginning of the round.

Acid Beam (Costs 3 Actions). The bonemantle deals a creature it can see 12d10 acid damage.

Acidic Blast (Costs 4 Actions). The bonemantle fires a blast of acid that takes the form of a 80 ft sphere, any creature caught in the sphere must make a DC 21 dexterity save, on a fail, the creature takes 8d12 acid damage, on a success, they take half has much.

Water Jet (Costs 2 Actions). The bonemantle fires an 100 ft line of water from it,  any target caught in the jet of water must make a DC 18 dexterity save (structures and objects instantly fail), on a fail, the target takes 4d12 bludgeoning damage and 4d12 slashing damage, on a success, they take half damage.

Teleportation. The bonemantle teleports to an unoccupied space within 500 ft of its current position.