Cursed Key

Cursed Key is a boss relic that grants 1 permanent Energy at the start of each turn but adds a Curse to your deck every time you open a non-boss chest. Chests appear regularly on the act map, so over a full run you will likely accumulate 3 to 5 Curses from chest openings with this relic.

Cursed Key has significant synergy potential. Du-Vu Doll gains Strength per Curse in deck, making Cursed Key a reliable way to accumulate Curses and Strength simultaneously. Darkstone Periapt gives Max HP per Curse obtained, also benefiting from the extra Curses. Blue Candle can exhaust Curses for 1 HP each, managing them in combat. Even without synergies, the extra Energy is powerful enough that the Curses are sometimes worth it — but avoiding chests is also a valid strategy if you don't want the deck pollution.