From Dragon Mania Legends Wiki
Yellow Dungeon Tokens are the currency for the Bottomless Dungeon feature introduced in Update 4.6. They are used for purchasing exclusive Dungeon Dragons, decorations and special offers found in the Dungeon Shop, purchasing certain islands, and the clearing of the obstacles from these islands.
This Token replaced the Blue Dungeon Tokens.
The only way to obtain Tokens is by winning battles in the Dungeon and then opening the Dungeon Chest(s). Dungeon Chests offer the most amount of Tokens, but they are not awarded if a Dragon is found inside the chest. The Dungeon resets every week, thus Tokens can be earned every day. Between 16 and 350 Dungeon Tokens are given per Dungeon Chest.