From Halopedia, the Halo wiki
The HAMMERHEAD armor set was originally created as a useable armor permutation for Halo Online, a cancelled Halo 3-derived multiplayer game developed by Saber Interactive and Innova Systems which was intended to be released exclusively in Russia for the PC. After the game's cancellation in 2016, the armor was one of six sets added to Halo 3 in The Master Chief Collection as part of the game's sixth season in 2021.
During the development of Halo Online, the HAMMERHEAD set was changed in the game's Beta build to use the name of "COMMANDO". The armor had a notably different backstory in this release, more similar to that of the COMMANDO-class Mjolnir found in Halo 4 and Halo 5: Guardians.
List of appearances