Maddening Presence.  Players are given sanity points equal to their wisdom plus intelligence modifier. Every time they fail a saving throw against one of Yogg'Saron's abilities, they lose a point of sanity. If a player is reduced to zero sanity upon failing a saving throw, they must make a DC20 wisdom saving throw. On a success, they regain one sanity point. If they fail, they are driven mad, and will no longer have control of their actions. They will then begin to attack any other creatures in their vision that are not Yogg'Saron until the encounter is ended. 

One-Thousand Tentacles. Yogg'Saron remotely controls countless tentacles within his domain. The amount of tentacles is equal to the number of players who have engaged him times five. The tentacles have 19 (3d10+8) hit points each, an AC of 11, and are immobile. They are immune to psychic damage and all conditions except for petrified and restrained. The Actions the tentacles can take each turn is defined in the Actions section. If all of Yogg'Saron's tentacles are defeated, Yogg'saron loses its ability to use legendary actions and must spend its next turn generating 1d4 more tentacles. These new tentacles spawn in any free spaces within 30ft of Yogg'Saron's primary body. Yogg'Saron can then use legendary actions again at the end of his next turn. Defeating a tentacle restores 1 sanity point to a random creature within 60ft of the fallen tentacle. Prioritizing those with the least sanity. Sanity cannot exceed the creatures wisdom plus intelligence modifiers. 

Mind Portal. Portals open up around Yogg'Saron's main body when he is reduced to zero hit points. Players must enter into the portals and defeat the brain of Yogg'Saron which has all of the same abilities, but now has a vulnerability to psychic damage. It is recommended that a separate map be used for this encounter. Tentacles can also now appear as various creatures of the DM's choosing instead of tentacles. Defeating Yogg'Saron's brain will forcefully eject players from his mind-space and they will reappear outside Yogg'Sarons main body. Players have 10 turns to defeat the main brain or be forcefully ejected from the mind-space. The main body will then regain hit points equal to the amount of lost sanity of all the creatures engaged with him times ten. Players will have to reduce Yogg'Saron to zero hit points again to return to the brain space. The Yogg'Saron within the mind-space retains all damage taken from previous attempts. 

Yogg'Saron's Abilities
Tentacle surge. Yogg'Saron Commands 1d4 tentacles to perform one of the available tentacle options.  

Ray of Insanity. Ranged Weapon Attack: +12 to hit, range 30/60 ft., 1d4 targets. Hit: 10 (1d20) psychic damage. Creatures hit by this spell must succeed on a wisdom saving throw of DC20 of lose a point of sanity. This ability cannot hit the same player twice in the same turn. Prioritizes creatures closest to Yogg'Saron. 

Tentacle Abilities

Grapple. The Tentacle attempts to grapple a creature within 10ft. It must succeed on a DC14 dexterity or strength saving throw or be restrained by the tentacle. Failure does not result in losing a point of sanity. 

Lesser Ray of Insanity. Ranged Weapon Attack: +8 to hit, range 30/60 ft., 1 target. Hit: 5 (1d10) psychic damage. Creatures hit by this spell must succeed on a wisdom saving throw of DC14 of lose a point of sanity. 

Flail. All creatures within 10ft of the tentacle must succeed on a DC14 dexterity saving throw or take 10 (2d8+3) bludgeoning damage and be knocked prone. 

 Schadenfreude. Yogg'Saron can heal as a bonus action equal to the amount of lost sanity between all creatures engaged with him.  

Old God's Legendary Resistance (3/day). upon successfully being hit by a spell attack or failing a spell saving throw, Yogg'Saron can cancel its effect. 

Yogg'Saron 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. Yogg'Saron regains spent legendary actions at the start of its turn. 

Tentacle surge. Yogg'Saron Commands 1d4 tentacles to perform one of the available tentacle options.  

Wheel of Madness (costs two actions). Roll a 1d4, the result is indicated below. If the option isn't possible to cast under certain circumstances, reroll the dice.

1. Cast Phantasmal Killer. DC20 Wisdom saving throw, failure results in creature becoming feared for one minute and must make wisdom saving throws at the end of each of the creature's turns or suffer 4d10 psychic damage.
2. Cast Blur on main body and all tentacles. Creatures have disadvantage on attack rolls made against anything effected by blur. Does not effect those with blindsight or truesight. Lasts for one minute or unless Yogg'Saron's concentration is broken (1d20+6)
3. Spawn Tentacles. Yogg'Saron spawns 1d4 new tentacles in any open spaces 10ft of any creature it is engaged with. 
4. Chaos Beam. Yogg'Saron fires a dazzling beam of energy from each of its eyes into everyone in front of him. All creatures engaged with Yogg'Saron or his tentacles who are not behind full cover and within 300ft must succeed on a DC20 wisdom saving throw or be affected by the spell Weird. Successive saving throw failures all reduce sanity.