Dark Matter Realm. Zero Two can only be fought in a realm of Dark Matter, where there is no solid surfaces and the fall is infinite. Only creatures with a fly speed by any means can move within this realm. Additionally, the Dark Matter Realm is entirely without light and so is entirely pitch black.

Innate Spellcasting. Zero Two's spellcasting ability is Intelligence (spell save DC 25, +20 to hit with spell attacks). Zero Two can cast the following spells innately and with no material components and are cast at the highest level possible:

Magic Missile, False Life, Ray of Frost Detect Thoughts, Fireball, Lightning Bolt, Chain Lightning, Sunbeam,
3/day: Counterspell,  Sunburst
1/day each: Wish, Power Word Kill, Meteor Swarm, Gate, Time Stop, Weird

Legendary Resistance (5/day). If Zero Two fails a saving throw, it may choose to succeed instead.

Dark Angel. Any of Zero Two's spells that deal Radiant damage always deal maximum damage. However, Zero Two also receives maximum damage roll from radiant damage. (This stacks with Vulnerability)

Omnipresent Magic. Zero Two and all spells it casts is immune to the effects of any form of magic dispelling, such as Dispel Magic or Antimagic Field.

Limited Magic Immunity. Unless Zero Two wishes to be effected, it is immune to spells of 4th level or lower and has advantage on savings throws of all spells and magical effects.

Everything will fall to Zero! If Zero Two's hit points are reduced to 500 or lower all of its once a day spells are recharged and Zero Two may now use actions in the "Mythic Actions" section. Award a party an additional 155,000 XP (310,000 XP Total) for defeating Zero Two after Everything will fall to Zero! activates.

Multicast. Zero Two may cast up to three times per turn when casting any spell that is not one of its once per day spells.

Stolen Stars. If any creature within 240 ft. casts wish, Zero Two may "steal" the spell, and casts the spell itself instead. 

Regain Distance (1/day). When Zero Two is hit by a Melee attack from within 15 ft., it may instantly teleport up to that creatures maximum movement speed + 50 ft. 

Zero Two can take 5 Legendary Actions. Only one legendary action option can be used at a time and only at the end of another creature's turn. If Zero Two has unspent legendary actions at the end of the round, it may spend and immediately use its remaining legendary actions. Zero Two regains spent legendary actions at the start of its turn.

Move. Zero Two may move up to half of its movement speed.

Innate Spellcasting (Costs 2 actions). Zero Two casts a spell.

If Zero Two has activated All will fall to Zero! in the last 12 hours, it can use the options below as legendary actions.

Fall to Zero (Costs 5 actions). Zero Two may nullify the effect of any spell or magical attack directed at it.

Gaze upon me! Do you fear me? (Costs 3 actions). Zero Two may look at any creature in a 240 ft. range and force the creature to look at it. The creature must make a DC 24 Wisdom saving throw or become Frightened of Zero Two.

Is nothing all we're meant for? (Costs 5 actions). Zero Two may invoke a feeling of meaninglessness in a creature and that creature must make a DC 24 Constitution saving throw or lose the will to continue for 1d4 turns. During this time, the only action the creature can use is the Help action.