Black Cyst

Amorphous. The Black Cyst can move through a space as narrow as 1 inch wide without squeezing.

Aura of Mind Erosion. Any hostile creature that starts its turn within 30 feet of the Black Cyst must make a DC 18 Intelligence saving throw. On a failed save, a creature suffers disadvantage on all Wisdom and Charisma saving throws until the start of the cyst's next turn.

Freedom of Movement. The Black Cyst ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled.

Innate Spellcasting. The Black Cyst's innate spellcasting ability is Charisma (spell save DC 18). It can innately cast the following spells, requiring no components:
At will: detect magic, protection from evil and good
2/day each: confusion, telekinesis
1/day each: enemies abound, fear

Psychic Mirror. If the cyst takes psychic damage, each creature within 10 feet of the cyst takes that damage instead; the cyst takes none of the damage. In addition, the cyst's location can't be discerned by magic.

Regeneration. The Black Cyst regains 10 hit points at the start of its turn. If it takes thunder damage, this trait doesn’t function at the start of its next turn. The Black Cyst dies only if it starts his turn with 0 hit points, doesn’t regenerate, and no Cystlings are within 60 feet of it. When it dies, it becomes an inactive Gem of Tharizdun.

Unyielding Essence. The Black Cyst is immune to any spell or effect that would alter its form, as well as those that would read its thoughts, determine if it is lying, or magically influence its thoughts or behavior.

Multiattack. The Black Cyst makes three attacks with its tentacles. It can also use Entropic Scream if it is available.

Tentacle. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 19 (3d8 + 6) bludgeoning damage plus 7 (2d6) necrotic damage, and the target is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the target takes 7 (2d6) necrotic damage at the start of each of its turns.

Entropic Scream (Recharge 5-6). The Black Cyst targets any number of creatures that it can see within 30 feet of it. Each target must make a DC 18 Wisdom saving throw, taking 27 (4d10 + 5) psychic damage on a failed save, or half as much damage on a successful one. If the saving throw fails by 5 or more, a target is also incapacitated until the end of its next turn.

Draining Essence. The Black Cyst targets a Cystling or creature affected by one of its spells within 15 feet of it. The creature must make a DC 18 Constitution saving throw, taking 9 (3d6) necrotic damage on a failed save, or half as much damage on a successful one. A willing creature can choose to fail the save, taking maximum damage. The Black Cyst's hit points increase by an amount equal to the damage taken.

Spawn Cystling. When the Black Cyst is damaged, it magically regurgitates a Cystling if it has at least 10 hit points. If it takes thunder damage, this trait doesn’t function until the start of its next turn. The Cystling emerges from its body and lands in the nearest unoccupied space. The creature is chaotic evil, sharing its consciousness with the cyst. The Black Cyst's current hit points and hit point maximum are reduced by the new Cystling's hit points.

The Black Cyst can take 3 legendary actions, choosing frm 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 Black Cyst regains spent legendary actions at the start of its turn.

Draining Essence (Costs 3 actions). The Black Cyst makes one Draining Essence Attack.

Tentacle. The Black Cyst makes one tentacle attack.

Transfer Essence (Costs 2 actions). The Black Cyst instantly swaps places with any Cystling within 60 feet of it or a creature affected by one of its spells.