Exile Feather

Exile Feather is a Rare relic that forms the backbone of exhaust-focused decks. Gaining 1 Energy each time a card is exhausted lets you play additional cards that turn, creating a powerful engine where exhausting cards fuels further plays. The classic combo is with exhaust-heavy cards and Soul Collector-type powers that draw on exhaust, generating both energy and cards simultaneously.

Cursed cards with Blood Vow become especially potent: play a curse for free via Blood Vow, exhaust it, gain energy from Exile Feather, and use that energy to play another card. Witch's Cauldron also synergizes by exhausting status cards at end of turn, generating energy for subsequent turns. Scroll of Rebirth adds another layer by replaying exhausted cards for free, which may exhaust again and generate more energy.

Note that Ethereal Lantern anti-synergizes, since it prevents Ethereal cards from exhausting, cutting off a potential energy source. Priority: always pick in any deck with exhaust effects; it is a cornerstone relic that enables an entire archetype.