Acerax

Assassinate.

During his first turn, Acerax have advantage on attack rolls against any creature that hasn´t take a turn. Any hit Acerax scores against a creature who hasn´t take his turn yet, is a critical hit.

Evasion.

If Acerax is subjected to an effect that allows him to make a Dexterity throw to take only half damage, he insted takes no damage if he succeeds on the saving throw, and only half damage if he fails.

Sneak attack(1/turn).

Acerax deals and extra 21 (6d6) damage when he hits a target with a weapon attack and has adventage on the attack roll, or when the target is within 5 feet of an ally of Acerax that isnt incapacitated.

Acrobatics Supreme (Rogue Augmetics).

Acerax can climb difficult surfaces, including upside down on ceilings, without needing to maje an ability check. Acerax has advantage on all Dex (Acrobatics) checks, and he can move through hostile creatures’ spaces without provoking opportunity attacks.

Mana Heart Aura (10 feet).

• Automatically dispel any level 4 or lower magic that enters the area.
• Level 5 or higher magic within the area has half duration and half damage or effect (before saving throws).
• Creatures in the aura are at a disadvantage in magic attacks or casting spells against Acerax.
• Acerax has the advantage in all saving spins against magic.

This aura counts as antimagic field-lite, but does not dispel Acerax’s own abilities.

Arcane Regeneration.

If Acerax starts his turn with at least 1 hit point, he regains 20 hit points. This regeneration stops only if he took radiant or psychic damage since his last turn.

Mage-Brain Reconstruction (Innate Spellcasting)

Acerax has fragments of a 20th-level archmage brain integrated into his own.
He can innately cast the following spells (spell save DC 20, +12 to hit):

1/day each:

• Dairy Disintegration. If the objective dies, turns into several slices of cheese.
• Fondue Storm. (incendiary cloud)
• Manchego Hold. (Hold person) 
• River of Fondue. (wall of fire)
• Cheddar Step. (Misti Step)
• Cheesepocalypse. (Meteor swarm)
• Cheese Transmutation: Transform Creature into Cheese

These spells require no components.

Multiattack

Acerax makes three attacks, in any combination of:

• Chrono-Vibrant Blade,

• Piercing Needle,

• Mana Burst.

Chrono-Vibrant Blade

Melee Weapon Attack: +12 to hit, reach 5 ft., one target.
Hit: 22 (3d8 + 8) slashing plus 7 (2d6) force.
Critical hits deal an additional 3d10 necrotic damage.

Piercing Needle

Melee or Ranged Weapon Attack: +14 to hit, reach 5 ft. or range 20/60, one target.
Hit: 17 (3d6 + 8) piercing.
If Acerax has advantage, the target must succeed a DC 20 Con save or become poisoned and slowed for 1 minute.

Mana Burst

Ranged Spell Attack: +11 to hit, range 60 ft., one target.
Hit: 27 (6d8) force damage.

Shadow Step (Teleport)

If Acerax is in dim light or darkness, he teleports up to 30 ft to another space he can see.

Rogue Overdrive (Recharge 5–6)

For 1 turn:

• His speed is doubled.

• He has advantage on all melee attacks.

• He doesn't provoke opportunity attacks.

Metallic Wrist Parry

Acerax reduces the damage of a melee attack that hits him by 1d10 + 8.

Parry.

When a creature within 30 ft make a attack (ranged or melee) against Acerax, Acerax attempts to interrupt it trowing a needle against the incoming proyectil/hit. He gets +5 armour class to this attack.

Acerax can take 3 legendary actions per turn, choosing from below.

Attack (1 action)

Acerax makes one attack with Chrono-Vibrant Blade.

Phantom Strike (2 actions)

Acerax strikes from an impossible angle.
The target has disadvantage on the saving throw, and
the attack deals an extra 14 (4d6) necrotic damage.

Quesomantic Discharge (2 actions)

Acerax releases an unstable wave of cheese-magic.
Creatures within 20 ft make a Dex save DC 20:

• Success: 15 (4d6) psychic

• Failure: 31 (8d6) psychic and their body partially “curdles,” reducing their speed by 10 ft until the end of Acerax’s next turn.

Core Reset (3 actions)

Acerax ends one negative condition affecting him (stunned, paralyzed, slowed, frightened, charmed, etc.).
His Regeneration also resumes even if suppressed.