The Bargainer Series by Laura Thalassa

Selling the complete Bargainer series by Laura Thalassa! Rhapsodic (Book 1): Callie is a siren burdened with magical IOUs (black beads) after making a deadly bargain. When the enigmatic Bargainer shows up to collect, the debt becomes a dangerous entanglement—and a threat to both their worlds.  A Strange Hymn (Book 2): Callie and Desmond are together but peace is elusive. As fae warriors vanish and suspicions grow, the pair journey into hostile realms, discovering that some bargains have consequences more severe than they ever imagined.  The Emperor of Evening Stars (Book 3): This installment takes a side or companion view within the same world, focusing on new characters and further expanding the supernatural stakes and romantic tensions.  Dark Harmony (Book 4): In the finale, old enemies awaken, magic spirals out of control, and Callie and Desmond must fight not just for each other, but for their realms. Death might no longer be the worst outcome.