Aspect of Tharizdun

Descent into Madness (Recharges on a Short or Long Rest). If the aspect would be reduced to 0 hit points or would otherwise be killed, it's current hit point total resets to 500, it's Unraveling Mind recharges, and it regains all spent Legendary Resistances. Additionally, the aspect can now use the options in the “Mythic Actions” section for 1 hour. Award a party an additional 155,000 XP (310,000 XP total) for defeating the Aspect of Tharizdun after its Descent into Madness activates.

Attack the Mind. The aspect's attacks, spells, and abilities that affect another creature are all entrenched in the mad god's insanity and falling under their effects causes one's mind to become afflicted with the same insanity suffered by Tharizdun. Any creature that is affected by or damaged by one of the Aspect's abilities must make a DC 18 Intelligence saving throw or become afflicted with Madness.

If a creature goes mad within line of sight of the Aspect of Tharizdun, roll on the Madness of Tharizdun table to determine the nature of the madness, which is a character flaw that lasts until cured. Additionally, on a failed save, the creature must also roll on the short-term Madness table in the Basic Rules compendium. If a creature fails this save 5 or more, then they must roll on the long-term madness table; if they fail by 10 or more, then they must roll on the Indefinite madness table. See the Dungeon Master’s Guide for more on madness.

Innate Spellcasting. The Aspect of Tharizdun's spellcasting ability is Intelligence (spell save DC 27, +17 to hit with spell attacks). The aspect can cast the following spells, with no material components required:

At will: Phantasmal Force, Mind Sliver, 
5/day: Tasha's Hideous Laughter, Invisibility, Nystul's Magic Aura
3/day: Tasha's Mind Whip, Confusion, Hold Monster
1/day: Time Stop, Hypnotic Pattern

Legendary Resistances (5/day). If the aspect fails a saving throw, she can choose to succeed instead.

Multiattack. The aspect makes three attacks: 2 touch attacks and 1 Madness Spike.

Touch. Melee Weapon Attack: +13 to hit, reach 5 ft., 1 target. Hit: 20 (3d10 + 4) psychic damage.

Madness Spike. Ranged Spell Attack: + 19 to hit, range 60 ft., 1 target. Hit: 36 (4d12 + 10) psychic damage, and the target must make a DC 20 Intelligence saving throw or suffer a random effect the madness table detailed in the Basic Rules compendium.

Unraveling Mind (Recharge 6). The aspect shudders as it's mind expands and attempts to unravel the mental capacities of all creatures around it. All creatures within 60 ft. of the aspect must make a DC 20 Intelligence saving throw or suffer 78 (12d12) psychic damage and randomly an effect from the madness table detailed in the Basic Rules compendium. 

Deceptive Movement. Whenever the aspect is hit by attack, it may use it's reaction to halve the damage taken and then move a distance up to its movement speed away.

Negate Spell (2/day). When the aspect sees a creature within 60 feet of it casting a spell, it attempts to interrupt the casting of the spell. If the creature is casting a spell using a spell slot of 8th level or lower, its spell fails and has no effect. If it is casting a 9th-level spell, it must succeed on a DC 27 Intelligence saving throw, or the spells fails and has no effect.

The Aspect can take 5 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. The Aspect regains spent legendary actions at the start of his turn.

Exploding Mind. The Aspect targets one creature within 30 feet of it that it can see and is currently under the influence of the Aspect's madness. That creature suffers 13 (3d8) psychic damage, and it is cured of all madness currently afflicting it as a result of the Aspect's presence.

Touch. The Aspect makes one Touch attack against a creature within range.

Slippery Dash (Costs 2 Actions). The Aspect moves up to half of it's walking speed in a direction of it's choosing without provoking attacks of opportunity.

If the Aspect's Descent into Madness trait has activated in the last hour, it can use the options below as legendary actions.

Spellcasting. The Aspect can cast one of it's at-will spells; this casting of the spell does not require concentration.

Maddening Speech (Costs 2 Actions). The Aspect chooses up to 6 creatures that can hear it, and attempts to muddle their minds. The target(s) must make a DC 18 Wisdom saving throw, or have their thoughts become muddled and difficult to sort for one minute. All affected creatures have a 1d8 penalty to attack rolls and ability checks whilst under the effects of the Aspect's Maddening Speech. A creature may repeat the saving throw at the end of their turn, ending the effects on themselves on a success.