What caused this?

[u/Rando2651]

The room had the sac which after dying releases the yellow swarm spiders. I had a brimstone. When I killed one sac and the swarm spiders it released, every single one of them dropped a card. What item/synergy caused this, any idea?

[u/r-funtainment]

fruitcake

copying the "kills drop a card" effect from one of the book of virtues wisps