Bottomless Dungeon Guaranteed Dragon
From Dragon Mania Legends Wiki
The Bottomless Dungeon offers Trainers an opportunity to acquire a different Event-Only Dragon each week. Acquiring the Event-Only Dragon can be done by participating in the weekly brawl and working through the map to the Epic Chest located at the end of the Dungeon Map. Once reached, the Trainer is guaranteed to receive the Event-Only Dragon once the chest has been opened 15 times.