The poison master - Liz Williams

On the planet Latent Emanation, human beings are the humblest class -- at the mercy of their mysterious alien rulers, the Lords of Night. Alivet Dee is a professional alchemist - and, unknowingly, a descendent of the famous Elizabethan mage Dr John Dee - who cannot help but question the Lords' rule ever since her twin sister was taken away to serve in their grim and enormous palace. She has saved every spare penny to pay her sister's un-bonding fee, but this plan is wrecked after one of her intricate potions goes wrong for a wealthy client. Subsequently, Alivet finds herself on the run for murder.Her only ally then seems to be a Poison Master from the planet Hathes. Arieth Ghairen is seeking an alchemist of Alivet's expertise to find the one drug that can harm the Lords and topple their hierarchy. Sequestered in Ghairen's fortress laboratory, lied to by both her new employer and his daughter's enigmatic governess, Alivet doesn't know who to trust or where to turn for answers. But, ever striving to rescue her sister, Alivet is driven relentlessly to hone her skills in time. Even though time itself is running out...