Ancient Dark aberration

Aberrant Ground. The ground in a 50-foot radius around the aberration is doughlike difficult terrain. Each creature that starts its turn in that area must succeed on a DC 20 Strength saving throw or have its speed reduced to 0 until the start of its next turn.

Gibbering. The aberration's many mouths constantly cry and moan while it can see any creature and isn't incapacitated. Each creature that starts its turn within 20 feet of the aberration and can hear the gibbering must succeed on a DC 20 Wisdom saving throw. On a failure, the creature can't take reactions until the start of its next turn and rolls a d8 to determine what it does during its turn. On a 1 to 4, the creature does nothing, takes 13 (3d8) psychic damage and is incapacitated until the effect ends. On a 5 or 6, the creature is charmed by the aberration plus it takes no action or bonus action and uses all its movement to move toward the aberration, begging to be devoured. On a 7, the creature makes a melee attack against a randomly determined creature within its reach or does nothing and takes 27 (6d8) psychic damage if it can't make such an attack. On an 8, the creature throws itself into the aberration's mouth and is devoured if it is size huge or smaller. If not the aberration can use its reaction to make one bite attack against the creature with advantage.

Legendary Resistance (3/Day). If the aberration fails a saving throw, it can choose to succeed instead.

Multiattack: The aberration makes three attacks with its tentacles and one with its bite.

Tentacle. Melee Weapon Attack: +14 to hit, reach 35 ft., one target. Hit: 24 (3d10 + 8) bludgeoning damage plus 14 (4d6) piercing damage. If the target is a creature, it is grappled (escape DC 22). Until this grapple ends, the target is restrained, takes 14 (4d6) piercing damage at the start of each of its turns, and the aberration can't use its tentacle on another target. The aberration has sixteen tentacles, each of which can grapple only one target. If a target is reduced to zero hit points by this attack it is devoured and absorbed by the numerous mouths along its tentacles into the aberration.

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one creature. Hit: 29 (6d6 + 8) piercing damage plus 7 (2d6) acid damage. If the target is Medium or smaller, it must succeed on a DC 22 Strength saving throw or be knocked prone. If the target is killed by this damage, it is absorbed into the aberration.

The aberration can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The aberration regains spent legendary actions at the start of its turn.

Tentacle Attack. The aberration makes one tentacle attack.

Bite Attack (Costs 2 Actions). The aberration makes one bite attack.

Devour (Costs 3 Actions). The aberration makes one bite attack against a creature it is grappling. On a hit the creature is restrained by the aberration's teeth. The creature must make a DC 22 Strength check at the start of its next turn to escape. Otherwise it is devoured by the aberration.