Knot in the Weave (Mythic; recharges after a Short or Long Rest). If Halaster would be reduced to 0 Hit Points, his current Hit Point total instead becomes 500 Hit Points, he regains one expended spell slot of each level, and his contingency spells activate. Additionally, he can now use the options in the "Mythic Actions" section for 1 hour.

 Equipment. Halaster wields the following equipment.
-- Shard of the Conjurer: Halaster’s Concentration on spells of the Conjuration school cannot be broken as a result of taking damage.
-- Horned Ring
-- Ring of Elemental Command (air)
-- Robe of Eyes

 Arcane Recovery (1/day). When he finishes a Short Rest, Halaster regains all spent spell slots of level 6 or lower.

Archwizard. Halaster can Concentrate on up to two spells at once.

Contingency. Halaster has two contingency spells in effect, both of which trigger when his Knot in the Weave ability activates.
-- Globe of invulnerability is cast on Halaster.
-- Bigby's hand is cast, appearing in a space of Halaster's choice within 10 feet of him in the Interposing Hand position.
(Halaster must Concentrate on both these spells once they are cast by contingency.)

Not Today, Asmodeus (5/day). If Halaster would fail a saving throw, he can instead choose to end Concentration on one spell he or an ally of his is currently Concentrating on. If he does so, Halaster succeeds on that saving throw instead.
[This ability replaces Legendary Resistance.]

Rejuvenation. If Halaster dies in Undermountain, he revives after 1d10 days, with all his Hit Points and any missing body parts restored. His new body appears in a random safe location in Undermountain. This can be prevented only by using a wish spell to trap Halaster's soul in the reassembled Undermountain Portrait of Memory.

Weave's Retribution. A creature that hits Halaster with a melee attack takes 2d6 Force damage.

Multiattack. Halaster uses Spellcasting and makes two Boom, Baby! attacks. He can replaces one of these attacks with a use of Chaos Obeys Me, if available.

Boom, Baby!. [melee or ranged spell attack] +17 to hit, reach 5ft or range 120ft, 1 target.
Hit: 3d6 Force damage plus 3d6 Thunder damage. The target must succeed on a DC 20 Strength saving throw or be pushed up to 10 feet away from Halaster.

Chaos Obeys Me (recharge 5-6). DC 26 Wisdom save; 120ft; 1 target Halaster can see.
Failure: The target is immediately affected by a Wild Magic Surge. Roll twice on the Wild Magic Surge table* and choose one of the results to apply.
Success: The target takes 1d12 Force damage as the threads of the Weave momentarily envelop them.

Spellcasting
Halaster's spellcasting ability is Intelligence.
Spell Attack +17 // Save DC 26

Halaster is a level 20+ spellcaster. He can cast magic missile and misty step at will. He can cast counterspell and dispel magic once each without expending a spell slot, but must finish a Short or Long Rest before he can do so again.

Cantrips (at will): fire bolt, light, mage hand, prestidigitation, shocking grasp

Level 1 (5 slots): absorb elements, mage armor, shield, thunderwave

Level 2 (4 slots): knock, mind spike, mirror image, misty step*, vortex warp*
Level 3 (4 slots): [Tooltip Not Found], dispel magic, fireball, fly, stinking cloud

Level 4 (4 slots): banishment, confusion, dimension door*, polymorph, summon construct*

Level 5 (4 slots): animate objects, Bigby’s hand*, cone of cold, dominate person, summon dragon*, wall of force

Level 6 (3 slots): chain lightning, disintegrate, globe of invulnerability, scatter, summon fiend*

Level 7 (3 slots): crown of stars, draconic transformation, forcecage, prismatic spray, reverse gravity

Level 8 (2 slots): maze, mind blank

Level 9 (2 slots): meteor swarm, wish

Quickened Caster. If Halaster cast a spell of level 1+ as part of an action on his turn, he can cast a cantrip as a Bonus Action.

Archmage Armor. [Trigger: When an attack roll misses Halaster or when Halaster succeeds on a saving throw against a spell that targets only him.]
Halaster attempts to turn his enemy's failure back onto them.
Response (attack roll): The attacker makes a new attack roll with themselves as a target. If the attack hits, it deals damage and/or confers its effects per its text.
Response (saving throw): The creature who triggered this ability must make the saving throw against the spell they used as though they were its target. Resolve the effects of a success or failure as per the text of the spell.

Legendary Action Uses: 4 (6 if his Knot in the Weave trait has activated in the last hour).

Immediately after another creature's turn, Halaster can expend one use to take one of the following actions. He regains all expended uses of Legendary Action at the start of each of his turns.

Arcane Maestro. Halaster casts a spell of level 5 or lower.

Catch Me If You Can. Halaster casts thunder step without expending a spell slot. He does not need to see the space he appears in.

Level Up!. DC 26 Charisma save; 90ft; 1 target Halaster can see.
Failure: The target is briefly banished, and their incorporeal form is magically hurled upward through all 23 layers of Undermountain in rapid succession. At the start of the target's next turn, they reappear in the space they left (or the nearest unoccupied space, if the space they left is occupied), immediately taking 23d4 Psychic damage.
Success: 23 Psychic damage only.
Whether the target succeeds or fails, Halaster can't take this action again until the start of his next turn.

If Halaster's Knot in the Weave Trait has activated in the last hour, he gains the Serious Business Time trait.

Serious Business Time. Halaster can take up to three Reactions per round, but only once each turn. His number of Legendary Actions increases from four to six.

Additionally, he gains the following two Legendary Action options.

Fraying Threads. Halaster uses his Chaos Obeys Me ability, regardless of whether it is recharged.

Undermountain Architect. Halaster changes the layout of Undermountain at his whim. Switch the battlefield to the next provided battle map. Place combatants in the same relative positions they were in on the previous map (or the nearest unoccupied space, if their previous space is occupied). Halaster can't take this action again until the start of his next turn.