Talk:Underworld/@comment-211.27.37.127-20160326071134/@comment-25669516-20160326071956

They regenerate and the end of each season but for Volcano only. For the other bosses you must fight and win to get the keys. You can check when a season will end by tapping the Dan icon in the left corner of the screen and opening up the leaderboard.

You can also buy the keys if need be for 30 gems.