Demonic Restoration. If Yeenoghu dies outside the Abyss, his body dissolves into ichor, and he gains a new body instantly, reviving with all his Hit Points somewhere in the Death Dells.

Legendary Resistance (4/Day, or 5/Day in Lair). If Yeenoghu fails a saving throw, he can choose to succeed instead.

Magic Resistance. Yeenoghu has Advantage on saving throws against spells and other magical effects.

Multiattack. Yeenoghu makes three Butcher attacks.

Butcher. Melee Attack Roll: +16 to hit, reach 15 ft. Hit: 28 (3d12 + 9) Force damage. If it’s his turn, Yeenoghu can cause the target to suffer one of the following additional effects, each of which he can apply only once per turn:

Confusion. The target can't take Reactions until the start of Yeenoghu’s next turn, and the target rolls 1d8 to determine what it does on its next turn: on a 1-4, the target does nothing; on a 5-6, the target takes no action or Bonus Action and uses all its movement to move in a random direction; on a 7-8, the target makes a melee attack against a random creature within its reach or does nothing if it can't make such an attack.

Force. The target takes an extra 19 (3d12) Force damage.

Paralysis. The target has the Paralyzed condition until the start of Yeenoghu’s next turn.

Spellcasting. Yeenoghu casts one of the following spells, requiring no Material components and using Wisdom as the spellcasting ability (spell save DC 22):

At Will: Detect Magic, Dispel Magic, Fear, Invisibility

1/Day: Teleport

Pack Rush. Each ally within 120 feet of Yeenoghu can take a Reaction to move up to half the ally's Speed without provoking Opportunity Attacks.

Rampage (Recharge 5–6). Immediately after dealing damage to a creature that is already Bloodied, Yeenoghu moves up to half his Speed, and he makes one Butcher attack.

Legendary Action Uses: 3 (4 in lair). Immediately after another creature's turn, Yeenoghu can expend a use to take one of the following actions. Yeenoghu regains all expended uses at the start of each of his turns.

Brand of Slaughter. Charisma Saving Throw: DC 22, one creature Yeenoghu can see within 120 feet. Failure: 36 (8d8) Necrotic damage, and the target is cursed. While cursed, the target can't benefit from the Invisible condition, its Speed decreases by 10 feet, and all Fiends within 120 feet of the target can sense its location regardless of interposing obstacles. Success: Half damage only. Failure or Success: Yeenoghu can't take this action again until the start of his next turn.

Ferocious Pounce. Yeenoghu moves up to half his Speed, and he makes one Butcher attack. On a hit against a Huge or smaller creature, Yeenoghu pushes the target up to 15 feet straight away from himself, and the target has the Prone condition.

Savage. Yeenoghu moves up to his Speed without provoking Opportunity Attacks. Each creature within 10 feet of Yeenoghu as he moves is targeted once by the following effect. Dexterity Saving Throw: DC 24. Failure: 25 (3d10 + 9) Acid damage. Success: Half damage. Failure or Success: Yeenoghu can't take this action again until the start of his next turn.