The Bargainer Series 4 Books Collection Set by Laura Thalassa (Rhapsodic, A Strange Hymn, The Emperor of Evening Stars & Dark Harmony):

Rhapsodic:
Callypso Lillis is a siren with a very big problem, one that stretches up her arm and far into her past. For the last seven years she's been collecting a bracelet of black beads up her wrist, magical IOUs for favors she's received. Only death or repayment will fulfill the obligations. Only then will the beads disappear.Everyone knows that if you need a favor, you go to the Bargainer to make it happen.

A Strange Hymn:
Siren and soulmate to the King of Night, Callypso Lillis survived the clutches of Karnon, the mad king, and his twisted prison. But the nightmare isn't over. Callie wears the physical reminders of her time as a captive, and mounting evidence suggests the Thief of Souls is still out there.When a fae celebration thrusts Callie and her mate, Desmond Flynn, into the Kingdom of Flora, they take their investigation with them.

The Emperor of Evening Stars:
In the beginning, there was darkness.Before he met Callie, before he became the Bargainer, there was Desmond Flynn, the bastard son of a scribe. A boy born to a weak mother, cursed with little magic, and destined to marry a slave.But fate had something else in mind.Till darkness dies.

Dark Harmony:
For Callypso Lillis, the fae magic that now runs through her veins is equal parts curse and good fortune. For the very thing that bonds her to Desmond Flynn, the King of the Night, also makes her vulnerable to the Thief of Souls, a man who wants to break the world ... and Callie along with it.But it's not just the Thief whose shadow looms over the Otherworld.