Avatar of the Corrupted Dreamer

Radiant Mirror. If the Avatar takes radiant damage, each creature within 20 feet of it takes that damage as well.

Magic Resistance. The Avatar has advantage on saving throws against spells and other magical effects.

Immutable Form. The Avatar is immune to any spell or effect that would alter its form.

Flanking Immunity. The manifold eyes of the Avatar gaze in every direction, making it immune to any benefit gained from flanking

Innate Spellcasting (Psionics). The Avatar's spellcasting ability is Charisma (spell save DC 23, +15 to hit with spell attacks). It can innately cast the following spells, requiring no components:

3/day each; confusion, eyebite, dispel magic

Multiattack. The Avatar makes three attacks: two with its barbed tentacles and one with its bite if it can. It then uses Mind Seed.

Barbed Tentacles. Melee Weapon Attack: +16 to hit, reach 30 ft., one creature. Hit: 25 (5d6 + 8) piercing damage, and the target is grappled (escape DC 19). Until this grapple ends, the target is restrained. The tentacles can grapple only one creature at a time.

Bite. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 30 (5d8 + 8) piercing damage. If the target is a Large or smaller creature, it must succeed on a DC 19 Dexterity saving throw or be swallowed by the Avatar. A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the worm, and takes 21 (6d6) psychic damage at the start of each of the Avatar's turns.

If the Avatar takes 30 damage or more on a single turn from a creature inside it, the worm 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 worm. If the Avatar dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 feet of movement, exiting prone.

Mind Seed (3/Day). The Avatar targets one humanoid currently grappled by it's tentacle, which must succeed on a DC 21 Intelligence saving throw or be cursed. The curse lasts until it’s removed by a remove curse or greater restoration spell.

The cursed target suffers 1 level of exhaustion every 24 hours, and finishing a long rest doesn’t reduce its exhaustion. If the cursed target reaches exhaustion  level 6, it doesn’t die; it instead becomes a thrall under the Dreamer's control, and all its exhaustion  is removed. Only the wish spell can free the thrall from this control.

Psychic Step. The Avatar magically teleports to an unoccupied space it can see within 60 feet of it.

Spell Mimicry (2/Day). Immediately after a creature the Avatar can see casts a spell of 5th level or lower, the Avatar tries to copy the spell. That creature must succeed on a DC 21 Charisma saving throw, or the Avatar immediately casts the same spell at the same level (+15 to hit with spell attacks, spell save DC 23), requiring no material components and choosing the spell’s targets.

The Avatar can take 4 legendary actions (5 in lair), choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The Avatar regains spent legendary actions at the start of its turn.

Tentacle. The Avatar makes one Tentacle attack.

Dispel. The Avatar casts dispel magic on one magical effect within range

Crushing Tentacle. The Avatar crushes one creature it is grappling. The grappled creature must make a DC 19 Constitution saving throw, taking 21 (4d6 + 8) force damage on a failed save, or half as much damage on a successful one.

Shatter Concentration (Costs 2 Actions). The Avatar targets a creature it is grappling. The target’s concentration on a spell it has cast or an ability it is maintaining ends, and the target takes 25 (4d12) psychic damage.

Disorienting Teleport (Costs 2 Actions). The Avatar teleports, along with any creatures it is grappling, to an unoccupied space it can see within 120 feet of itself. A creature that is grappled and any creatures within 10' of the space the Avatar left take 2d10 psychic damage.

Psychic Whispers (Costs 3 Actions). The Avatar emits dreadful whispers in a 60-foot-radius sphere centered on itself. Each creature in the sphere that isn’t an Aberration must make a DC 21 Wisdom saving throw, taking 33 (6d10) psychic damage on a failed save, or half as much damage on a successful one.