Ustroth, Harbinger of Chaos

Legendary Resistance (3/Day). If Ustroth fails a saving throw, he can choose to succeed instead.

Unyielding Essence. Ustroth is immune to any spell or effect that would alter his form, as well as those that would read his thoughts, determine if he is lying, or magically influence his thoughts or behavior.

Psychic Strain. Any creature psychically damaged by Ustroth has a 1/4 chance to gain 1 level of insanity.

Negative Energy Aura. Any non-undead creature that starts its turn within 30 feet of Ustroth takes 14 (4d6) necrotic damage. Furthermore, non-undead creatures can't regain hit points and do not benefit from resistance to necrotic damage while within 30 feet of the Ustroth. A non-undead creature that were to magically regain hit points must instead make a DC 25 Constitution saving throw. On a failed save, the creature takes necrotic damage equal to 1/2 the hit points it would have regained.

Psychic Persistence. Ustroth bypasses psychic immunity and all special traits that defend against psychic damage. On the other hand, any creature affected by this bypasses Ustroth's psychic immunity.

Cruel Mercy. If a creature dies by Ustroth's hand he can revive them as a aberration under his control with identical stats and abilities as the killed creature by taking an amount of damage equal to the new creature's hit-points. This can only be done if the creature agrees to it or fails a DC 18 Charisma saving throw.

Enslavement. If a creature who has been made insane by Ustroth is not cured within 7-level of insanity days the creature will begin to worship and serve Ustroth as a god. This creature will then attempt to seek out Ustroth so he can turn them into an aberration.

Untethered to Reality. Ustroth can concentrate for five minutes to cast Plane Shift or Dimensional Door with no components. Or, he can use an action to cast Teleport. This ability is unaffected by any anti magic effects.

Psychic Storm. Ustroth can deal 13 (2d12) psychic damage to up to 5 targets. On a successful DC 25 Intelligence saving throw the creature will take half damage. On an unsuccessful saving throw the creature becomes prone for 2 rounds and takes an extra 7.5 (1d8+3) psychic damage.

Corrupted Strike. Melee Weapon Attack: +18 to hit, reach 5 ft., 1 target. Hit: 28.5 (3d12 +9) necrotic damage plus 12.5 (1d12+6) bludgeoning damage. Target must roll DC 23 Intelligence or DC 20 Constitution save (target's choice) or have disadvantage on all Intelligence and Wisdom saving throws until after a long rest.

Psychic Glare. Ranged Weapon Attack: +12 to hit, range 50 ft., 1 target. Hit: 33 (6d10) psychic damage. Target must succeed on a DC 24 Intelligence saving throw or be stunned for one round. All saving throw proficiencies are nullified next time the target makes a saving throw.

Power Absorption. Ustroth can kill an aberration under his control from up to 150 feet away to gain one of its abilities, special traits, or resistances for 1 day. It is up to the DM to decide how abilities/traits heavily tailored to one creature might be adapted to Ustroth.

Reflect Energy. When a spell is successfully used on Ustroth he can chose to take 4d6 radiant damage and become unable to use any actions or legendary actions for an amount of rounds equal to the level of the spell in order to cause the spell to fail and instantly cast any spell of lower than or equal level in response.

Ustroth can take 3 legendary actions, 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. Ustroth regains spent legendary actions at the start of his turn.

Show True Form (Costs 3 Actions). All targets who can currently perceive the Ustroth must make a DC 24 Wisdom saving throw or gain 3 levels of insanity. Creatures who pass this check become frightened and only stand to gain 1 level of insanity if the ability is used again on them. This resets after a long rest.

Summon Aberration. Ustroth can summon any number of aberrations under his control whose CRs add up to a maximum of 3 multiplied by the number of legendary actions used for this ability. Leftover CR can be used next time Ustroth uses this ability.

Use Power (Costs 1 Action). Ustroth uses one of his actions.

Energy Flow (Costs 0 Actions). Ustroth transfers hit-points between himself and an aberration under his control or between aberrations under his control. 1/2 of the hit-points are lost during the transfer. The aberration(s) in question must be less than 150 feet away.