Cultist of Leviathan, Punished Ascendant

Traits:
Leviathan’s Punishment. 
This form exists only because Leviathan wills it. The cultist cannot be banished, polymorphed, or possessed, and ignores effects that would alter its form.

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

Eldritch Abyss
The cultist’s weapon attacks and spells ignore resistance to necrotic, psychic, and force damage.

Many-Eyed Horror
At the start of each of the cultist’s turns, it chooses one creature it can see within 60 feet: That creature has disadvantage on the first attack roll or saving throw it makes before the start of the cultist’s next turn.

Actions
Multiattack
The cultist makes three attacks, any combination of Abyssal Blade and Tentacle of the Deep, or replaces one attack with Eldritch Cataclysm (if available).

Abyssal Blade
Melee Weapon Attack: +15 to hit, reach 10 ft., one target
Hit: 25 (3d8 + 7) slashing + 18 (4d8) necrotic damage

If the target is Large or smaller, it must succeed on a DC 22 Strength saving throw or be knocked prone.

Tentacle of the Deep
Melee Weapon Attack: +15 to hit, reach 20 ft., one target
Hit: 21 (3d8 + 7) bludgeoning damage + 13 (3d6) psychic damage
Target is grappled (escape DC 22). While grappled, the target is restrained and takes 18 (4d8) necrotic damage at the start of each of its turns.

Eldritch Cataclysm (Recharge 5–6)
The cultist releases a surge of abyssal power in a 60-foot cone.

Creatures in the area must make a DC 24 Dexterity saving throw:

• Failure: 72 (16d8) necrotic and knocked prone

• Success: Half damage and not knocked prone

The area becomes lightly obscured by writhing shadows until the end of the cultist’s next turn.

Bonus Actions
Hex of the Deep (Recharge 4–6)
The cultist curses one creature it can see within 90 feet for 1 minute:

• The cultist deals +10 necrotic damage to the cursed target

• The target has disadvantage on saving throws against the cultist’s abilities

Reactions
Abyssal Parry
When hit by a melee attack, the cultist adds +5 to AC, potentially causing the attack to miss. If it misses, the attacker takes 15 (3d10) psychic damage.

Legendary Actions
The cultist can take 3 legendary actions, choosing from the options below. Only one option can be used at a time and only at the end of another creature’s turn.

Tentacle Lash
Makes one Tentacle of the Deep attack.

Eldritch Step
The cultist teleports up to 30 feet to an unoccupied space it can see.

Gaze of Leviathan (Costs 2 Actions)
One creature within 60 feet must succeed on a DC 24 Wisdom saving throw or be stunned until the end of its next turn.

Abyssal Collapse (Costs 3 Actions)
The cultist releases crushing pressure in a 20-foot radius centered on itself. Creatures of its choice must make a DC 24 Constitution saving throw or take 45 (10d8) force damage and be knocked prone; half damage on a success.

Mythic Action
When reduced to 0 HP, the cultist does not die. Instead:

• It regains 250 HP

• All tentacle attacks deal an additional 2d8 necrotic damage

• Leviathan speaks directly into the minds of the party