Boon of Immortality. Achilles doesn't age and is immune to being magically aged. In addition, he doesn’t need to breathe, drink, eat or sleep.

Dark Devotion. Achilles has advantage on saving throws against being charmed.

Focused Caster. Achilles has advantage on Constitution saving throws made to maintain concentration on spells.

Pact Magic. Achilles' spellcasting ability is Charisma (spell save DC 20, +12 to hit with spell attacks). He can cast the following spells:

At will: mind sliver, eldritch blast (4 beams, +5 to damage), message, mage hand, invisibility, levitate

3/per Short Rest (5th-level): counterspell, dimension door, raulothim's psychic lance, diminish magic, banishment, mind raider, detect thoughts, greater invisibility, mystic surge

1/per Long Rest each: teleport, consume memory, power word pain, dominate monster 

Psionic Defense. Achilles adds his Charisma modifier (+6) to his Armor Class (included in statblock).

Multiattack. Achilles uses Unravel Minds (if available) and makes two Psychic Spike attacks. Alternatively, he can cast one spell from his Pact Magic trait and make a single Psychic Spike attack.

Psychic Spike. Ranged Spell Attack: +12 to hit, range 90 ft., one target. Hit: 22 (5d8) psychic damage.

Unravel Minds (Recharge 5-6). Achilles targets up to three creatures he can see within 60 feet of him and forces each of them to make a DC 20 Intelligence saving throw. On a failed save, a creature takes 45 (10d8) psychic damage and can’t take reactions for the next minute. Moreover, on each of its turns for the same duration, it must choose whether it gets a move, an action, or a bonus action; it gets only one of the three. On a successful save, a creature takes half damage and suffers no other effects. A lesser restoration spell or similar magic ends this effect early. 

Psionic Retaliation. As a reaction to taking damage, Achilles can force the attacking creature to make a DC 20 Intelligence saving throw. On a failed save, the creature takes 14 (4d6) psychic damage and is pushed 15 feet away from him.