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
Unlock: Achieve level 3 in all of the Spartan Company Mastery Commendations' Kill Commendations for the armor, and achieve level 5 in all of the Spartan Company Mastery Commendations' Kill Commendations for the helmet.
Halo 5: Guardians REQ cards
List of appearances