Xbox has rolled out the "Mythic Achievement" badge to Preview Program members. Players can earn this badge by completing all achievements initially available in a game; achievements added later via DLC or expansion content do not count toward this requirement.
This means that players may already meet the requirements for the "Mythic Achievement" badge even without completing content added to the game later. The eligibility also extends back to Xbox 360-era titles, so players who completed the initial achievements of older games may see corresponding records in their profiles.
The "Mythic Achievement" badge is primarily used to showcase a player's gaming history. Currently, this feature is available to Xbox Preview Program members.