Unarmored Defense.  While not wearing any armor, Achillum's Armor Class equals 10+ his Dexterity Modifier + his Constitution modifier.

Reckless Attack.  Attack rolls by and against Achillum have advantage as his rage spills into fierce desperation.

Danger Sense. Achillum has advantage on Dexterity Saving throws against effects he can see such as traps and spells.

Brutal Critical. When determining the extra damage for a critical hit with a melee attack, Achillum rolls three additional dice.

Indomitable Might. If Achillum's total for a Strength check is less than his Strength score, he can use that score in place of the total.

Feral Instinct. When rolling initiative, Achillum does so with advantage. Further, he cannot be surprised in combat.

Legendary Resistance (3/day).  If Achillum fails a saving throw, he can choose to succeed instead.

Relentless Rage. If Achillum drops to 0 hit points and doesn't die outright, he can make a DC 0 Constitution saving throw. If he succeeds he drops to 1 hit point instead.  Each time he uses this feature, the DC increases by 1.  When he finishes a short or long rest, the DC resets to 0.

Multiattack. Achillum makes five attacks: three with Wrathbringer and two with his horns.

Wrathbringer. Melee Weapon Attack: +17 to hit, reach 10 ft., one target. Hit: 24 (1d12 + 18) slashing damage plus 6 (1d12) psychic damage. A target hit by this attack must succeed on a Wisdom saving throw or be frightened of Achillum for one minute. 

Horns. Melee Weapon Attack: +14 to hit, range 5 ft., 1 target. Hit: 20 (2d6 + 13) piercing damage.

Shadow Step. Achillum steps through nearby shadows, reappearing from another shadow within 30 feet.

Retaliation. When Achillum takes damage from a creature within 10 feet of himself, he can use his reaction to make a melee weapon attack against that creature.

Achillum can take 3 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.  Achillum regains spent legendary actions at the start of his turn.

Gore. Achillum makes a horns attack.

Shadow Step. Achillum uses his Shadow Step bonus action.

Sweep (Costs 2 Actions). Achillum sweeps Wrathbringer in a wide arc, making an attack roll against each creature within 10 feet of him.