Beasts Within (Mythic; Recharges after a Short or Long Rest). If Yeenoghu is reduced to 0 hit points, he doesn’t die or fall unconscious. Instead, he becomes incredibly feral and unpredictable. Yeenoghu immediately gains 333 hit points. Until this trait recharges, he gains the following benefits:

• At the end of a turn, if he dealt damage since the start of his turn, Yeenoghu regains 15 hit points and 10 temporary hit points.
• Whenever he deals damage, he has advantage on his next attack roll and creatures attacking Yeenoghu have advantage to hit him.
• When Yeenoghu kills a creature, 2d4 gnolls erupt out from the corpse. They act immediately after his turn in combat and treat Yeenoghu as an ally.
• He has resistance against bludgeoning, piercing, and slashing damage from magical attacks.
• Any critical hit against Yeenoghu becomes a normal hit, as if he were wearing adamantine armor.
• The effects of Yeenoghu's Claw are applied to each of Yeenoghu's attacks instead of once each turn.

Additionally, he can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 185,000 XP (330,000 XP total) for defeating Yeenoghu after his Beasts Within trait activates.

Blood Frenzy. Yeenoghu has advantage on melee attack rolls against any creature that doesn't have all its hit points

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

Lord of Gnolls. Yeenoghu has advantage on attack rolls, saving throws, and Charisma and Wisdom (Insight) checks against gnolls and hyenas. All gnolls and hyenas have disadvantage on attack rolls and saving throws against Yeenoghu. Gnolls and hyenas are unwilling to attack Yeenoghu and obey his orders unquestionably (no action required), even if they are influenced by magic.

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

Pack Lord. Any gnoll and/or hyena within 120 feet of Yeenoghu of his choice has advantage on attack rolls, saving throws, and can't be charmed or frightened. In addition, If Yeenoghu isn't incapacitated, any creature with the Rampage trait/bonus action of his choice can make a Bite attack as a bonus action as long as they are within 120 feet of Yeenoghu.

Ravenous Aura. Yeenoghu constantly radiates an otherworldly hunger out from him in a 60 ft. aura, driving his allies forward. When Yeenoghu and creatures of his choice within the aura reduce a creature to 0 hit points with a melee attack, Yeenoghu can use his reaction to move up to half his speed and make a melee weapon attack. Additionally, each hostile creature that starts its turn within 60 ft. of Yeenoghu of his choice takes 14 (4d6) piercing damage. A creature can't regain hit points while it is within 60 feet of Yeenoghu.

Siege Monster. Yeenoghu deals double damage to objects and structures.

Yeenoghu's Claw (1/turn). When Yeenoghu hits a creature with a melee weapon attack, he can add one of the following effects:

• The attack deals an extra 5 (1d10) force damage.
• The target must succeed on a DC 23 Wisdom saving throw or immediately use its reaction to make a melee attack against a random creature, other than Yeenoghu, within its reach.
• The target must succeed on a DC 23 Constitution saving throw or have disadvantage on the next saving throw it makes before the start of your next turn.

Multiattack. Yeenoghu makes three Flail attacks.

Flail. Melee Weapon Attack: +21 to hit, reach 15 ft., one target. Hit: 22 (2d12 + 12) force damage. Yeenoghu scores a critical hit on a roll of 19 or 20 with the flail. When Yeenoghu takes the Attack action on his turn, he can make a sweeping flail attack. This targets up to 3 creatures within 15 feet of Yeenoghu, and he makes a single attack roll for this sweep. If it’s his turn, Yeenoghu can cause a target to suffer one of the following additional effects, each of which he can apply only once on each creature: 

• Confusion. The target must succeed on a DC 21 Wisdom saving throw or be affected by the confusion spell until the start of Yeenoghu’s next turn.
• Force. The target takes an extra 21 (3d12 + 3) force damage.
• Paralysis. The target must succeed on a DC 21 Constitution saving throw or be paralyzed until the start of Yeenoghu’s next turn.

Bite. Melee Weapon Attack: +18 to hit, reach 10 ft., one target. Hit: 20 (2d10 + 9) acid damage and Yeenoghu regains 10 hit points. If the target is a creature, Yeenoghu can choose to grapple it. The target is grappled (escape DC 23). Until this grapple ends, the target is restrained, and Yeenoghu can't Bite another target.

Impaling Wrath (Recharge 5-6). Yeenoghu touches the ground and causes a 10-foot-tall wall of iron spikes to erupt in a 120-foot line that is 20 feet wide. When the wall appears, each creature in its area must make a DC 23 Dexterity saving throw. On a failed save, a creature takes 70 (20d6) piercing damage and is impaled. A impaled creature is restrained by the wall (escape DC 20), and takes 13 (3d8) piercing damage at the start of each of its turns as long as it is restrained. On a successful save, a creature takes half as much damage and isn't impaled. The wall of iron spikes is permanent and can only be destroyed with a magical attack that deals 35 or more damage. The wall blocks line of sight and counts as difficult terrain for all creatures except Yeenoghu. A creature that isn't Yeenoghu takes 9 (2d8) piercing damage for each 5 feet it moves in the wall's area.

Spellcasting. Yeenoghu casts one of the following spells, requiring no material components and using Charisma as the spellcasting ability (spell save DC 22, +14 to hit). When Yeenoghu uses danse macabre on dead gnolls, he can raise them as gnoll witherlings instead of zombies or skeletons:

At will: detect magic, Hideous Laughter, Ray of Enfeeblement, Wrathful Smite, Vampiric Touch, Contagion, Danse Macabre, Locate Creature, Command (3rd level), polymorph (into hyenas only)

3/day each: dispel magic, fear, Erupting Earth, invisibility, Haste, Staggering Smite, Earthquake, Blade Barrier

1/day: teleport, Destructive Wave, mass polymorph (into hyenas only)

Yeenoghu can take 2 bonus actions on his turn.

Cursed Insults (Recharge 5-6). Yeenoghu hurls savage and horrific insults laced with curses at a target he can see within 60 feet of him. The target must succeed on a DC 22 Charisma saving throw or be cursed. While cursed, the creature is under an effect similar to a Bestow Curse spell (Yeenoghu chooses the curse), except it does not require concentration and lasts until it is lifted by Remove Curse or similar magic cast at 7th level or higher.

Curse of Yeenoghu. When Yeenoghu hits a creature with a damage dealing spell, he can inflict a curse on it. Only one creature can be cursed in this way at a time. A cursed creature has disadvantage on Wisdom saving throws and attack rolls and loses one of it's damage resistances if it has one. The curse ends after target takes damage or makes attack action. After the curse ends, target is immune to this effect for 24 hours.

Demonic Hunger. Yeenoghu can ravenously consume the corpse of a Small or Medium creature over the course of 1 minute, during which time he can do nothing but feast on the creature’s dead flesh. At the end of this time, the corpse is destroyed, and he regains 11 (2d10) hit points. Yeenoghu can also take 5 minutes to consume the corpse of a Large creature and regain 22 (4d10) hit points, 30 minutes to consume the corpse of a Huge creature to regain 33 (6d10) hit points, or 1 hour to consume the corpse of a Gargantuan creature to regain 55 (10d10) hit points.

Incite Madness. Yeenoghu emits a maddening howl. Creatures of his choice within 30 ft. of him must make a DC 23 Wisdom saving throw. On a failed save, they immediately use their reaction to make a melee attack against a random target within their reach. If they have no targets within their reach, they lose their reaction on that turn.

Rampage. When Yeenoghu reduces a creature to 0 hit points with a melee attack, he moves up to half his speed and makes one Bite attack. If the attack hite, he can make another Bite attack.

Yeenoghu can make 2 reactions each round.

Insatiable Strike. When Yeenoghu drops to 0 hit points, he can make a melee weapon attack against a creature within range. If the attack hits, Yeenoghu regains 23 hit points.

Yeenoghu 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. Yeenoghu regains spent legendary actions at the start of his turn.

Charge. Yeenoghu moves up to his speed.

Swat Away. Yeenoghu makes one Flail attack. If the attack hits, the target must succeed on a DC 24 Strength saving throw or be pushed up to 15 feet in a straight line away from Yeenoghu. If the saving throw fails by 5 or more, the target is also knocked prone.

Savage (Costs 2 Actions). Yeenoghu makes a separate Bite attack against each creature within 10 feet of him.

If Yeenoghu is a Mythic encounter, he can use the options below as legendary actions for 1 hour after using Beasts Within.

Callout. Yeenoghu calls his next attack, announcing it aloud. If his next weapon attack hits, it is automatically a critical hit.

Reinforcements. Yeenoghu vomits a 1d4 gnoll fangs of Yeenoghu into adjacent unoccupied spaces.

Sweeping Flail (Costs 2 Actions). Yeenoghu makes a flail attack against each creature within 15 feet of him. On ahit, a creature must also succeed on DC 25 Dexterity saving throw or be knocked prone.

Yeenoghu's Corpse Curse (Costs 3 Actions). Yeenoghu taints up to five corpses within 120 ft. of himself with his demonic will, imbuing the targets with his Curse, which attracts a spectral scavenger beast to appear and consume the corpse and transform into a shoosuva. On each of his turns, Yeenoghu can use a bonus action to mentally command any shoosuva he has made with this action if the creature is within 120 ft. of him (if Yeenoghu controls multiple shoosuva, he can command any or all of them at the same time, issuing the same command to each one). Yeenoghu decides what action the creature will take and where it will move during its next turn, or he can issue a general command, such as guard a particular chamber or corridor. If Yeenoghu issues no commands, the creature only defends itself against hostile creatures. Once given an order, the creature continues to follow it until the task is complete.