Bosses have a chance to drop a card which represent them. For now, the cards are used to improve and customize Necromancer's Shoggoth and Sorcerer's summoned creature, as well as used in a mini-game duel with other players. Cards may also be leveled up by feeding them other cards.
Cards Level-up (Enhancement)
It is possible to improve a card by feeding it other cards. To do so, you need to open the Card Enhancement window while in Rest Mode.
A Necromancer can insert Animal, Plant and Mutant type cards into their Necronomicon to improve and customize the Shoggoth's stats.
Similarly to Necromancers, a Sorcerer may insert Demon type cards into their Grimoire to improve, customize and change the appearance of their summoned creature, using Summoning skill.
A player can start a card mini-game duel with another player. The game is chance based, the system will automatically pick certain criterias, such as "tallest monster", "flying type", "dark element", and whichever cards fits those criterias the best, win.
List of Cards
You can consult the list of all known boss cards on TOSBase's database.