Dark Albion is unlocked once you have completed the credits level of the main game. Dark Mistpeak is the third level of this second playthrough.
After you have fully upgraded a character you will move to use the inside track of the upgrade mini-game, where each tile gives you a temporary 'perk'. Before playing Dark Mistpeak play something short like the sledge or minecart ride, and when you land on one of the inside tiles buy the perk. Then immediately start a match on Dark Mistpeak. Simply complete the level and the achievement will unlock.