Xbox begins rolling out Mythic Achievements to Insiders
Xbox is testing Mythic Achievements with Insiders: a permanent badge for finishing every original achievement in a game. Past completions, including Xbox 360 titles, have been applied retroactively.

Xbox has started rolling out Mythic Achievements, a new way to mark games you've fully completed, to Xbox Insiders from September 29. Xbox Wire describes it as one of the most player-requested features.
What happened
You earn a Mythic Achievement by unlocking all of a game's original achievements. Achievements added later through DLC or updates aren't required, and once you have the status it's yours to keep.
Xbox says it has already applied Mythic status to eligible games players finished in the past, Xbox 360 titles included, so some profiles may show new badges straight away.
How you track progress
Each game's achievement list gets a progress bar showing how close you are to Mythic, with an option to focus on the original achievements you still need. Add-on achievements are labelled separately.
Insiders also get a new Gaming tab on their profile showing achievements, Gamerscore milestone badges, completed games and Mythic Achievements. Friends' tabs can be viewed too.
Why it matters
Achievement hunters get a clear finish line per game, and old completions may count immediately.