Avatar of Entropy

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

Unstoppable. The Avatar of Entropy does not have a Constitution score. The Avatar of Entropy automatically succeeds on any Constitution saving throws, checks, or skills and is immune to any effects that rely on Constitution.

Undead Endurance. All of the Avatar of Entropy's current and future hit dice become d12s and it always rolls the maximum amount when determining maximum hit points gained. In addition, the Avatar of Entropy gains 3 maximum hit points per hit die.

Lifesense. The Avatar of Entropy is aware of the location, size, and type of all living things within 1000 feet of it. In addition, the Avatar of Entropy knows if the creature is suffering from any ailments (such as diseases, injuries, or conditions) or has any ongoing spells cast upon it and generally how close it is to death as well as its general emotional state (anger, fear, love, etc.).

Innate Spellcasting. The Avatar of Entropy's innate spellcasting ability is Charisma (save DC 26, +19 to hit with spell attacks). The Avatar of Entropy can cast the following spells, requiring no material components. At Will: Arcane Hand (9th), Cone of Cold (9th), Dispel Magic (5th), Wall of Ice (9th), Elemental Bane (necrotic/cold; 9th), Nondetection (self), Invisibility (Self), Plane Shift (self), Speak with Dead, Teleport
5/Day: Haste, Mislead, Harm

Legendary Resistance (3/Day). If the Avatar of Entropy fails a saving throw, it can choose to succeed instead.

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

Perception Filter. The Avatar of Entropy's nature defies that of the physical world and it destroys all properties and processes that would be used to perceive it. The Avatar of Entropy cannot be detected by nonmagical means unless a creature is already aware of it and of its current location. The Avatar of Entropy can activate or deactivate this trait on any or all creatures as a bonus action.
This has various effects:

Enemies that don't possess truesight or a magical sense cannot become aware of the Avatar of Entropy's presence unless another creature that is aware of the Avatar uses an action to describe it and its location or by using a magical effect. Each time a non-deific creature loses any sense of the Avatar of Entropy, it forgets it (For example, a creature looking at the Avatar looks away and forgets what it looks like).
The Avatar of Entropy can attempt to hide from any and all creatures, even while in plain sight.
When the Avatar of Entropy becomes hidden from a creature, that creature must use a magical sense to detect the Avatar of Entropy or otherwise be made aware of the Avatar of Entropy by another creature or by a magical effect.

Deconstruction Aura. Other creatures within 30 feet of the Avatar of Entropy that are alive can't regain hit points. Any other creature that starts its turn within this aura takes 3d6 necrotic damage. Any creature that dies within this aura can't be resurrected by any means other than a Wish or True Resurrection spell.

Counter-Turn. The Avatar of Entropy and all undead within 30 feet of it are immune to effects that turn undead. In addition, when an enemy attempts to turn undead while within 30 feet of the Avatar of Entropy, that creature must make a Charisma save versus the Avatar of Entropy's spell save DC. On a failure, the creature suffers the effects of its own ability as if it was undead and Avatar of Entropy was using the ability (use the Avatar's stats and traits).

Withering Weapons. The Avatar of Entropy's weapon attacks are magical. When the Avatar of Entropy hits with any weapon, the weapon deals an extra 6d8 necrotic damage (included in the attack). In addition, the target must succeed on a DC 26 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Multiattack. The Avatar of Entropy chooses any two of the following:
- Two Greatsword attacks
- Death Ray
- Icy Wail

Greatsword. Melee Weapon Attack: +19 to hit, reach 10 ft., one target. Hit: 24 (4d6 + 10) slashing damage plus 27 (6d8) necrotic damage. The target must succeed on a DC 26 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Death Ray. The Avatar of Entropy forces one creature within 120 feet of it that it can see to make a DC 26 Constitution saving throw. On a failed save, the target takes 88 (13d8 + 30) necrotic damage, and its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. On a successful save, the target takes half as much damage, and its hit point maximum is not reduced. The target dies if this effect reduces its hit point maximum to 0.

Icy Wail (Recharge 6). The Avatar of Entropy lets out a withering, icy cold wail. Any creature within 120 feet of the Avatar of Entropy that can hear the wail must succeed on a DC 26 Constitution saving throw or gain one level of exhaustion. In addition, all creatures within 120 feet of the Avatar of Entropy must make a DC 26 Dexterity saving throw. On a failure the creature is frozen in a block of ice for 8 hours.
Creatures frozen in ice are restrained and unconscious and can't breathe. At the end of each of its turns, a creature frozen in ice takes 12d8 cold damage. Each ice block has AC 12 and 30 hit points. When the ice is reduced to 0 hit points, the creature is no longer frozen.

Power Word: "Purge". The Avatar of Entropy reaches into the soul of a creature and attempts to destroy it utterly. The Avatar of Entropy must make a melee spell attack (Charisma) against a creature. On a hit, the creature suffers the effects of the Avatar of Entropy's Withering Weapons. After the effects resolve, if the crature has 50 or fewer hit points, the creature is slain and its soul is destroyed utterly. This prevents any form of resurrection as the spell will not have a soul to retrieve.
After the Avatar of Entropy uses this ability, it can't be used again until after a short rest.

The Avatar of Entropy can take 3 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 of Entropy regains spent legendary actions at the start of its turn.

Greatsword. The Avatar of Entropy attacks with its Greatsword.
Death Ray (Costs 2 Actions). The Avatar of Entropy uses its Death Ray.
Icy Wail (Costs 3 Actions). The Avatar of Entropy uses its icy wail.