Avatar, The King In Yellow

Incorporeal Movement. The avatar can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.  Once per turn, creatures it moves through take 11 (2d10) psychic damage when the avatar passes through their space.

Psychic Absorption.  When the avatar is dealt psychic damage, it does not take any damage, and instead gains temporary hit points equal to the psychic damage dealt.

Legendary Creature.  Legendary Creatures have advantage on initiative checks, proficiency in all saving throws, and can make extra reactions each round (but only 1 per turn).  Spells or abilities that remove reactions (other than conditions such as stunned, incapacitated, et.c) instead prevent the legendary creature from using reactions on that turn (not the whole round), except for Fade from Reality.

Writhing Tentacles.  Dexterity Saving Throw:  DC 17, all creatures in a 10 foot emanation centered on the avatar.  Failure: 7 (1d8+3) bludgeoning damage plus 10 (3d6) psychic damage, and the creature becomes confused (as the spell) until the start of the avatar's next turn.  Success:  half damage and the target is not confused.

Grant Mark.  Wisdom saving throw: DC 17, one creature within 5 feet of the avatar.  Failure:  the target is marked for 1 minute.  While marked, the target suffers a 3 (1d6) penalty to attack rolls, ability checks, and saving throws when it can see the avatar.

Possession. One humanoid or corpse that has been marked by the avatar that it can see within 5 feet of it must succeed on a DC 19 Charisma saving throw or be possessed; the avatar then disappears, and the target is incapacitated.  On its turn, the avatar chooses the targets actions.

While possessed, the avatar has access to all knowledge and abilities of the possessed target, and uses the physical ability scores of the target.  It gains 25 temporary hit points, and any damage dealt to the avatar deals half of the damage to the possessed creature.  The possession ends when the avatar loses its temporary hit points.

When the possession ends, the avatar reappears in an unoccupied space within 5 feet of the body. The target is immune to this avatar's Possession for 24 hours after succeeding on the saving throw or after the possession ends.

Spellcasting.  the avatar casts one of the following spells, using charisma as its spellcasting ability and needing no verbal, somatic, or material components.

At Will: command, detect thoughts, fear, modify memory, tasha's mind whip (4th level), telekenesis

1/day:  power word: pain, synaptic static, teleport

Etherealness. The avatar enters the Ethereal Plane from the Material Plane, or vice versa. It is visible on the Material Plane while it is in the Border Ethereal, and vice versa, yet it can't affect or be affected by anything on the other plane.

Legendary Reactions.  The creature can make 3 reactions per round, with an additional reaction for every 2 player characters over 4.

Command Awe.  Trigger:  a creature within 60 feet ends its turn.  Response - Wisdom Saving Throw:  DC 17, the avatar casts command (grovel) on the triggering creature.

Viral Madness.  Trigger:  a creature deals damage to the avatar.  Response - Constitution saving throw: DC 17.  Failure:  the target has the poisoned condition until the start of the avatar's next turn.  While poisoned, the target is Confused (as the spell).  Failure by 5 or more:  the target is poisoned, and all creatures within 30 feet must also make a saving throw against Viral Madness.

Ethereal Waves (in ethereal plane only).  Trigger:  A creature within 60 feet ends its turn.  Response - Wisdom Saving Throw: DC 17, creatures in a 30 foot cone.  Failure: 5 (1d10) psychic damage and targets fall prone.

Fade from Reality.  Trigger:  the avatar fails a saving throw and is under a condition or effect.  Response:  The condition or effect ends and the avatar enters the ethereal plane.