From Halopedia, the Halo wiki
The Athlon helmet features an inner layer with a magnetorheological dampening system specifically crafted for Spartan physiology.
The armor is used during live-fire War Games exercises.
Halo 5: Guardians
Available skins: Champion, Cynisca, Iccus, Lycinus and Varazdat
The Athlon suit was first shown in the Halo 5: Guardians Multiplayer Beta announcement trailer. The permutation is part of Breakout, a new game mode in Halo 5: Guardians.
Halo 5: Guardians skins
Halo 5: Guardians REQ cards
List of appearances