Raid difficulty is a setting controlled by the raid leader that determines the difficulty of the encounters for 10-man or 25-man raids. There is a Normal mode and a Heroic mode for each size. Warlords of Draenor introduced Mythic mode 20-man raids. As of Legion, Mythic mode will have some variability based on [Mythic Keystones].[citation needed] As of Warlords of Draenor, Flexible Raid mode was effectively replaced by a flexible Raid Finder mode. Burning Crusade raids only have fixed 10-man or 25-man sizes and 20-man Mythic mode introduced.