From Halopedia, the Halo wiki
The Achilles suit's neural interface is equipped with a savage AI that gnaws at the mind of its user, easily leading to excessive violence in combat.
Only select Spartans who have mastered themselves are allowed to requisition the Achilles set, as unskilled wearers will quickly lose themselves to uncontrolled rage at the hands of the suit's embedded AI.
Halo 5: Guardians
Achilles was added into Halo 5: Guardians in the Infinity's Armory update. It is unique among the Halo 5 armor sets in that it is one of the few that can only be unlocked via specific in-game achievements, as opposed to the requisition system.
Achilles was known internally as "A102_S000 Waypoint Companies Armor" during development. The armor was initially accidentally unveiled with the Cartographer's Gift update, when some glitched REQ cards were available to view with the Achilles name - though only placeholder imagery.
Concept art and Artwork
List of appearances