The poison master - Liz Williams

On the planet of Latent Emanation, humans form the lowest class, at the mercy of strange and decadent alien rulers: the Lords of Night. The young alchemist Alivet has chafed under the Lords' domination ever since her twin sister was abducted to serve in their palace. But, having saved every penny to rescue her, Alivet's plans are wrecked when she finds herself on the run for murder.

Her only hope now seems to be a mysterious stranger named Ari Ghairen, the Poison Master from the planet Hathes. Ghairen needs an alchemist of Alivet's expertise to compound the one drug that can free her human kin by destroying the Lords. Sequestered in Ghairen's fortress laboratory on Hathes, lied to by both her new ally and the enigmatic governess he employs, Alivet doesn't know whom to trust or where to turn. But if she doesn't find some answers soon, she will lose everything.