The Poison Master - Liz Williams 2004 Sci-Fi Paperback

On a distant world ruled by an alien race, humanity is enslaved, having forgotten its own past and condemned to have no future--until one woman is offered a choice that could restore humanity's freedom. The Poison Master On the planet of Latent Emanation, humans are the lowest class, at the mercy of their mysterious alien rulers, the Lords of Night. But Alivet Dee, an alchemist, can't help but question the Lords' rule ever since her twin sister was taken to serve in their palace. Alivet saves every penny to pay her sister's unbonding fee, but her plan is destroyed when one of her potions kills a wealthy client--and Alivet finds herself wanted for murder. Her only hope is the darkly attractive man who may have engineered her downfall but who still offers her a last chance of salvation. A Poison Master from the planet Hathes, Arieth Mahedi Ghairen needs an alchemist of Alivet's expertise to find the one drug that can take down the Lords--and free the universe from their rule. Sequestered in Ghairen's fortress laboratory, lied to by both her new ally and his daughter's enigmatic governess, Alivet doesn't know whom to trust or where to turn for answers. But driven to undo her sister's fate, Alivet races to hone her skills in time--even as time runs out.