Gameplay

Mystery Achievement

From Halopedia, the Halo wiki

Mystery Achievement

Mystery Achievement is an achievement in Halo: The Master Chief Collection. It is unlocked by activating .fortune Easter Egg. The Easter egg will appear if the player types ".fortune" as a name almost anywhere in the game, including map variants, game types, loadouts, clan tags, or even Service IDs. The achievement is worth 5 gamerscore.