Madness Incarnate. Yogg'saron is immune to all spells and conditions that affect and target minds and thoughts. If a spell is used to try and see Yogg'saron's thoughts, caster rolls a DC 24 Wisdom saving throw. On success, nothing happens. On a failed save, the caster takes 32 (8d6 + 8) psychic damage and they can not make an action on their next 3 turns.

Blood of the Void. Each time Yogg'saron is hit with slashing or piercing damage, he bleeds a greenish-black blood. When a creature touches the blood, they role a DC 20 Wisdom saving throw. On a success, nothing happens. On a failure, the creature loses all rational thought, speaking gibberish, and attacking anything it can see. ||| If the blood is allowed to dry and harden, it becomes a very strong metal that can be harvested and used to create strong armor (DM choice on how strong). |||

The Manifestation of Nightmares. When a creature is put to sleep by a spell, or naturally, if that creature can dream, they must make a DC 22 Wisdom saving throw. On a success, that creature is able to dream normally and can take wake up. On a failure, that creature is trapped in the worst nightmare that it's mind can think of, and they can not wake up. For every creature trapped, Yogg'saron summons a 9th-level Nightmarish Copy. The Nightmarish Copy has all the senses, abilities, spells, and powers of the creature being copied.

The Beast With a Thousand Maws. After Yogg'saron Grapples a creature with its Tentacle, if it is still grappled the on its next turn, Yogg'saron attempts to swallow the creature whole. If the creature is swallowed, the creature is stuck in Yogg'saron's stomach and takes 18 (6d6) acid damage. If the creature ends its turn in the stomach, the creature takes the same amount of acid damage at the start of the creature's next turn.

The Mind of Insanity. When Yogg'saron loses half of its health, it gains immunity to all forms of damage that aren't dealt by a god-like being. To do this, Yogg'saron opens a gateway directly into its mind. Creatures inside Yogg'saron's mind take 9 (3d6) psychic damage at the end of their turn. Creatures inside Yogg'saron's mind will be able to see and attack the manifestation of Yogg'saron's brain. When the brain loses all hit points, all creatures are ejected from the mind and Yogg'saron loses its immunity and is stunned for two turns.

Multiattack. Yogg'saron can make three attacks. Two Tentacle attacks and One Strike of Madness.

Tentacle. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 24 (3d10 + 9) bludgeoning damage. Target creature must make a DC 18 Strength saving throw, or become Grappled.

Strike of Madness. Ranged Weapon Attack: +14 to hit, reach 60 ft., one target. Hit: 34 (4d12 + 10) psychic damage. Victim must make a DC 20 Wisdom saving throw, or lose their mind to insanity. On a fail, the victim loses all rational thought and attacks anything it can see. Each turn the victim can make a DC 20 Wisdom saving throw to regain their mind.

Yogg'saron can take 5 legendary actions, choosing from the options below. Only two legendary actions can be used at the end of another creature's turn. Yogg'saron regains spent legendary actions at the start of its turn.

Surrender to Madness. Target creature must make a DC 20 Wisdom saving throw, or be thrown into an insanity induced rage, causing the creature to attack the nearest creature that isn't Yogg'saron. Creature can make a DC 19 Wisdom saving throw on its turn to try and break the rage.

Tentacle. Yogg'saron makes one Tentacle attack.