Emperor of Evening Stars by Laura Thalassa

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.
From the barren caves of Arestys to the palace of Somnia to the streets of earth, this is how Desmond Flynn, a fairy who began with nothing, became the Emperor of Evening Stars.