Regret

Regret is a Curse card that is Unplayable and causes you to lose HP equal to the number of cards in your hand at the end of each turn. In a standard 5-card hand, holding Regret means losing 5 HP when the turn ends — a brutal penalty. Decks that draw many cards or hold large hands are penalized most severely. The only mitigation is playing as many cards as possible before end of turn to reduce hand size, but Regret itself cannot be played to lower the count. It has no upgrade.