Every monster has its own card; they drop rarely (see each monster's page). Three ways to use one:
- Slot it into gear of the type it fits (at Brann or from the bag); taking it out again costs gold.
- Register it in the Collection book (menu → Collection) for a permanent bonus of about a third of the slotted one. The card is used up.
- Master a skill (Lv 6): some skills need a specific card.
The frame tells the rarity: gold/bronze normal · silver rare · violet mini boss · crimson-gold MVP. All of them are on the Cards page.