Critical Mass. For every cumulative one hundred hit points of damage Achilles takes, his critical hit range is increased by one (to a maximum of ten)

Dark Regeneration. If Achilles has at least 1 hit point, he regains 60 hit points at the start of his turn. If he was dealt radiant damage in the previous round, he may only regain 30 hit points in this manner.

Legendary Resistance (5/Day). If Achilles fails a saving throw, he can choose to succeed instead.

Limited Magic Immunity. Unless he wishes to be affected, Achilles is immune to spells of 5th level or lower. He has advantage on saving throws against all other spells and magical effects.

Magic Weapons. Achilles’ weapon attacks are magical.

Void Touched. Achilles is permanently under the effects of the shadow of moil spell, except that it does not grant resistance to radiant damage, does not require concentration, and he heals for the amount of damage dealt as the shadows siphon the life from their target.

Innate Spellcasting. Achilles' spellcasting ability is Charisma (spell save DC 25). Achilles can innately cast the following spells, requiring no material components:

5/day each: Necrostrike (Not a spell, works as smite. 5d6 necrotic damage), Deathgrip (200 ft. CON save or pulled to within 5ft. of Achilles), 

3/day each: finger of death, disintegrate, Sanguine Strike - next melee weapon hit = crit, healed for total amount of damage dealt.

2/day each: steel wind strike, demiplane, power word stun, circle of death (8th level)

1/day each: psychic scream, antimagic field, maddening darkness, circle of death (9th level)

Multiaction. Achilles can do one of the following (can't use the same option twice in a row):

• three melee weapon attacks
• two melee weapon attacks and one spell
• one melee weapon attack and two spells

Razael's Demise. Melee Weapon Attack: +19 to hit, reach 5ft., one target. Hit:  24 (4d6 +10) slashing damage plus 14 (4d6) psychic damage.

Shadow Spear. As a reaction to being targeted by a ranged spell, spell attack, or magical effect, Achilles hurls a ten foot spear of swirling malevolent energy at the attacker forcing them to make a Charisma saving throw. On a failed save, the target takes 14(4d6) necrotic damage and is silenced until the end of their next turn. On a successful save the spear misses.

Die Together. As a reaction to being critically hit, Achilles momentarily links his life force to that of his attacker. The total damage of the attack is split between himself and the attacker with each taking fifty percent. 

Achilles 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. Achilles regains spent legendary actions at the start of his turn.

Attack. One melee weapon attack with Razael's Demise.

Move. Up to his movement speed maximum.

Cast a Spell (Costs 3 Actions). Achilles casts a spell from his list of innate spells.

Embrace The Void. (Auto trigger at 50% health) Achilles slams his palm onto the ground. Each creature within a 100-foot-radius must make a Dexterity saving throw. A creature takes 20d6 necrotic damage and 20d6 cold damage on a failed save, or half as much damage on a successful one. Regardless of whether or not they succeed, each creature is pushed back 20 feet and knocked prone. The terrain within the area of affect of the spell becomes desecrated ground, reducing healing from any source and radiant damage by half for all hostile creatures.