The Legion expansion introduced the Mythic+ dungeon difficulty system to World of Warcraft. Mythic+ transformed ordinary five-player dungeons into scalable, repeatable challenges with increasing difficulty and improved rewards.
Players begin with a Mythic Keystone that identifies a dungeon and a level. Completing the run within the time limit can upgrade the keystone, while higher levels increase enemy health and damage and introduce affixes that change how the dungeon plays. This created a competitive endgame loop for groups that wanted an alternative to raiding.
Mythic+ became available during Legion and was expanded through later seasons and expansions. Battle for Azeroth did not introduce the system; it continued and developed the Legion model. Warlords of Draenor had Mythic dungeons, but not the timed, keystone-based Mythic+ format. Shadowlands likewise inherited the system rather than originating it.