From Halopedia, the Halo wiki
Hell's Janitor is a multiplayer medal in Halo 3, Halo: Reach and Halo 5: Guardians awarded for killing ten Zombies in a row without dying in the Infection game variant.
It is represented by a bronze five-pointed star with a deformed green skull on a green background in the center.
This medal is "officially" obtainable in the Double EXP Weekend Playlist Living Dead to be viewed on Bungie.net.
The remodeled version in Halo: Reach.
The medal in Halo: The Master Chief Collection.