As Seth, your Grimoire contains 7 sin cards, one for each of the 7 deadly sins. This new card type can be played for free. You spend an action to play a sin card, resolve its effect, and then perform a different action.
The effect of a sin card is minor, but after that effect, the sin card is attached to your character skill card. For an action, you can rain a hail of magical bullets down on your enemies, dealing collateral damage based on the number of sin cards you have collected.
