Avatar of Emptiness

Anathematic Secrecy. The avatar can't be targeted by any divination magic or perceived through magical scrying sensors.

Brightest Star in a Perfect World. When the avatar dies, it explodes in a burst of entropic energy. Each creature within 300 feet of it must make a DC 27 Dexterity saving throw, taking 90 (20d8) force damage on a failed save, or half as much damage on a successful one.

Chaotic Form. The form of the avatar constantly shifts, making it difficult to hit. Attack rolls against it are made at disadvantage, and it doesn’t require air, food, drink, or sleep, Additionally, the avatar is immune to disease, madness, and effects that would reduce it’s maximum hit points.

Entropic Chaos. The avatar's weapon attacks are magical. Each time the avatar attacks, it can choose to deal acid, cold, fire, lightning, or thunder damage instead of the normal damage of the attack.

Entropic Resilience. The avatar has advantage on saving throws against spells and other magical effects. If the avatar rolls a natural 20 on a saving throw against a spell that targets only it (not in an area of effect), the spell has no effect on the avatar, and it targets the caster instead, using the slot level, spell save DC, attack bonus, and spellcasting ability of the caster.

Entropic Shield. At the start of each of its turns, the avatar rolls a d6 as an entropic shield forms around it, granting it immunity to one type of damage until the start of its next turn, as detailed in the table below:

Innate Spellcasting (Psionics). The avatar's innate spellcasting ability is Charisma (spell save DC 27). It can innately cast the following spells, requiring no components:

At will: plane shift, teleport (without error)

Legendary Resistance (5/Day). If the avatar fails a saving throw, it can choose to succeed instead.

Paradise Parasite (Mythic Trait, Recharges after a Short or Long Rest). When the avatar would be reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, it regains 615 hit points, it activates its Brightest Star in a Perfect World trait, and it regains any expended uses of Legendary Resistance. Additionally, it can now use the options in the “Mythic Actions" sections for 1 hour. Award the party an additional 155,000 XP (310,000 XP total) for defeating the avatar after its Paradise Parasite trait activates.

Reaper’s Due. If it dies, the avatar returns to life in 1d6 days and regains all its hit points.

Unyielding Essence. The avatar is immune to any spells or effects 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 avatar makes two Entropic Claw attacks and one Entropic Bolt attack. It can replace one of its attacks with a usage of Cataclysm.

Entropic Claw. Melee Weapon Attack: +19 to hit, reach 20 ft., one target. Hit: 36 (4d12 + 10) slashing damage plus 18 (4d8) force damage.

Entropic Bolt. Ranged Weapon Attack: +19 to hit, range 120 ft., one target. Hit: 45 (10d8) necrotic damage.

Cataclysm (Recharge 5-6). The avatar uses one of the following attacks:

Absolute Zero Outburst. The avatar creates a 60-foot radius storm of magical energy centered on itself. Each creature in that area must make a DC 27 Constitution saving throw, taking 45 (10d8) cold damage and 45 (10d8) necrotic damage on a failed save, or half as much damage on a successful one.

Bloody Fountain. The avatar unleashes a torrent of entropic blood in a 120 foot cone or in a 240 foot line that is 10 feet wide. Each creature in that area must make a DC 27 Strength saving throw, taking 45 (10d8) piercing damage and 45 (10d8) force damage on a failed save, or half as much damage on a successful one.

Planet Smash. The avatar hurls planetoids at up to seven points within 200 feet that it can see. Each creature within a 40-foot radius of one of those points must make a DC 27 Dexterity saving throw, taking 45 (10d8) bludgeoning damage and 45 (10d8) fire damage on a failed save, or half as much damage on a successful one. A creature in the area of more than one planetoid is affected only once.

Entropic Surge. The avatar overcharges the next weapon attack it makes until the start of its next turn with entropic energy, dealing an extra 13 (2d12) acid, cold, fire, lightning, or thunder damage (Entropos chooses the damage type) on a hit.

Entropic Wrath. When a spell is cast within 120 feet of the avatar, it can use its reaction to cast a duplicate of that spell, using the slot level, spell save DC, attack bonus, and spellcasting ability of the caster. The avatar chooses the new targets.

The avatar can take 5 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 avatar regains spent legendary actions at the start of its turn.

Drift. The avatar makes a Wisdom (Perception) check or moves up to half its speed.

Flailing Frenzy (Costs 2 Action). The avatar makes one entropic claw or entropic bolt attack. It can instead magically teleport, along with any equipment it is wearing or carrying, up to 120 feet to an unoccupied space it can see.

Recover (Costs 3 Actions). The avatar regains 30 hit points.

Shatter Reality (Costs 3 Actions). The avatar alters the fabric of reality in a 60-foot radius around itself until the start of its next turn. The area becomes affected by either the darkness, forbiddance, mirage arcane, or reverse gravity spell (the avatar chooses which).

Journey’s End (Costs 5 Actions). The avatar unleashes a surge of raw entropic power. Each creature within 60 feet of it must make a DC 27 Dexterity saving throw, taking 39 (6d12) force damage on a failed save, or half as much damage on a successful one, and is pushed 30 feet away from it.

If the avatar's Paradise Parasite trait has activated in the last hour, it can use the options below as legendary actions.

Entropic Affinity (Costs 2 Actions). The avatar gains immunity to acid, cold, fire, lightning, or thunder damage (its choice) until the start of its next turn.

Antishadow Onslaught (Costs 3 Actions). The avatar makes one entropic claw and one entropic bolt attack.

Paradise Reclaimed (Costs 5 Actions). The avatar unleashes two effects from the Lair Actions Table (see Description).