Legendary Resistance: (3/day)

Magic Weapons: Yogg-Saron's attacks are considered magical for overcoming resistances and immunities.

Magic Resistance: Yogg-Saron has advantage on all magical Saving Throws.

Regeneration: At the start of its turn, Yogg-Saron regenerates 50Hp. This ability doesn't work if Yogg-Saron was hit by a critical hit.

Manipulator of Fate: While Yogg-Saron has legendary resistances, any roll, that isn't Yogg-Saron's, that has advantage is just a straight roll. 

Wild Aura. Any spell, that isn't Yogg-Saron's, that is cast within 50ft of Yogg-Saron has a 25% chance of going wrong. (For example, spell hits different target, spell doesn't go off,  an inverse effect happens, wild magic happens, a completely different spell goes off with varying targets. (DM's Discretion))

 Spellcasting. Yogg-Saron is a 20th level spellcaster. Its spellcasting ability is [Intelligence] (Spell Save DC 25, +19 to hit with spell attacks). Yogg-Saron has the following spells prepared:

• Cantrips (At will): Toll the Dead, Sword Burst, Shocking Grasp, Sapping Sting, Ray of Frost, Fire Bolt, Eldritch Blast, Chill Touch.
• 1st (At will): Armor of Agathys, Arms of Hadar, Bane, Chaos Bolt, Charm Person, Command, Dissonant Whispers, Guiding Bolt, Hellish Rebuke, Hex, Inflict Wounds, Sleep, Tasha's Hideous Laughter, Witch Bolt.
• 2nd (At Will): Blindness/Deafnesss, Cloud of Daggers, Crown of Madness, Darkness, Detect Thoughts, Enlarge/Reduce, Enthrall, Flaming Sphere, Hold Person, Maximilian's Earthen Grasp, Melf's Acid Arrow, Mind Spike, Moonbeam, Ray of Enfeeblement, Scorching Ray, Shatter, Silence, Suggestion.
• 3rd (At Will): Animate Dead, Bestow Curse, Call Lightning, Counterspell, Dispel Magic, Erupting Earth, Fear, Fireball, Haste, Hunger of Hadar, Hypnotic Pattern, Lightning Bolt, Melf's Minute Meteors, Protection from Energy, Pulse Wave, Sleet Storm, Slow, Spirit Guardians, Tidal Wave, Vampiric Touch.
• 4th (At Will): Arcane Eye, Banishment, Blight, Confusion, Control Water, Death Ward, Dimension Door, Evard's Black Tentacles, Fire Shield, Gravity Sinkhole, Hallucinatory Terrain, Ice Storm, Phantasmal Killer, Polymorph, Shadow of Moil, Sickening Radiance, Storm Sphere, Summon Greater Demon, Wall of Fire.
• 5th (At Will): Awaken, Banishing Smite, Bigby's Hand, Cloudkill, Cone of Cold, Conjure Elemental, Conjure Volley, Contagion, Destructive Wave, Dominate Person, Enervation, Flame Strike, Geas, Immolation, Insect Plague, Modify Memory, Negative Energy Flood, Scrying, Synaptic Static, Wall of Force.
• 6th (5 slots): Blade Barrier, Chain Lightning, Circle of Death, Disintegrate, Flesh to Stone, Gravity Fissure, Harm, Mass Suggestion, Mental Prison, Ottiluke's Freezing Sphere, Otto's Irresistible Dance, Sunbeam, Wall of Ice.
• 7th (4 slots): Crown of Stars, Divine Word, Finger of Death, Fire Storm, Plane Shift, Power Word Pain, Prismatic Spray, Reverse Gravity.
• 8th (3 slots): Dark Star, Dominate Monster, Earthquake, Feeblemind, Illusory Dragon, Incendiary Cloud, Maddening Darkness, Maze, Power Word Stun, Reality Break, Sunburst, Tsunami.
• 9th (2 Slots): Blade of Disaster, Imprisonment, Mass Polymorph, Power Word Heal, Power Word Kill, Prismatic Wall, Psychic Scream, Ravenous Void, Weird.

Multiattack: Yogg-Saron can do two tentacle attacks and one bite attack.

Tentacle: Melee Weapon Attack, +16 to hit, reach 20ft, one target. Hit (4d8+10) bludgeoning damage. The target is then grappled (escape DC 18), the target is then restrained while grappled.

Bite: Melee Weapon Attack, +18 to hit, reach 10ft, one target. Hit (4d10+10) piercing damage.

Swallow: Yogg-Saron makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside Yogg-Saron, and it takes 21 (6d6) acid damage at the start of each of Yogg-Saron's turns. Yogg-Saron can have only two creature swallowed at a time.
If Yogg-Saron takes 30 damage or more on a single turn from a creature, Yogg-Saron must succeed on a DC 20 Constitution saving throw at the end of that turn or regurgitate the creature, which falls prone in a space within 10 ft. Yogg-Saron. If Yogg-Saron dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 ft. of movement, exiting prone.

Spellcasting: Instead of the Multiattack, Yogg-Saron can cast two spells, one At Will and one Non-At Will. These are picked randomly from his list.

The Wheel of Yogg! (Recharge 6): The Wheel of Fate is spun! What it lands on is only up to chance. May the odds be in your favor. Roll a d10 to determine which effect from the Wheel of Yogg! is triggered. (Yes, Yogg-Saron is included as a target for these effects.)

1. 10 different Wild Magic Surges go off at once.
2. Roll a d4, that many random targets have a -10 to all attacks, ability checks, and saving throws until their next turn.
3. One creature, picked randomly, has vulnerability to all damage until their next turn.
4. Roll a d4, that many random targets skip their next turn.
5. Roll a d12, cast that many spells, determined randomly, from Yogg-Saron's spell list at random targets. (Yogg-Saron does not expend any spell slots for this effect.)
6. One random creature is to be the target of all attacks and spell effects until their next turn.
7. Target one random creature. That creature must make a DC 22 CON Save. On a fail, the target's maximum and current HP is set to 1 until their next turn. On a success, the target's maximum and current HP is halved until their next turn.
8. Reversal Aura. Until Yogg-Saron's next turn any effect that deals damage heals, and any effect that heals deals damage.
9. Roll a d4, that many random targets are affected by the Plane Shift spell, auto hit. The targets must then succeed on a DC 23 CHA Save or be teleported to a random plane.
10. Cast 6th Level Disintegrate at random targets until one creature dies.

Rigged Wheel: If Yogg-Saron is targeted by an effect from the Wheel of Yogg! it can use its reaction to target a different creature besides itself instead. Yogg-Saron can only use this reaction while it has legendary resistances.

Yogg-Saron can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature's turn. Yogg-Saron regains legendary actions at the start of its turn.

Bite (1 Action). Yogg-Saron makes one bite attack, if it is against a grappled creature, Yogg-Saron can go for a Swallow instead.

Tentacles (1 Action). Yogg-Saron can make two tentacle attacks.

Spellcasting (2 Actions). Yogg-Saron can cast one of its spells.

Wheel of Yogg! (3 Actions). Yogg-Saron can use its Wheel of Yogg! ability. Yogg-Saron cannot use this legendary action twice in a row.