Mythic Achievements for Xbox Insiders will officially roll out starting today, and Microsoft has provided more detail on how its new milestone system works. Designed to celebrate the biggest Xbox achievements that a user has unlocked throughout their PC and console history–and similar to PlayStation Platinum Trophies–Mythic Achievements are earned whenever all original achievements for a game have been earned. Microsoft added that achievements added later through DLC or updates aren’t required, and once you’ve earned Mythic status, it’s yours to keep…