The Potion of Death was a potion created by contacts of Lucy Saxon's family to counter the Potion of Life.
History
Aware that the Saxon Master was likely to return, Lucy Saxon had contacts of her family make the Potion of Death to counteract the Master's Potion of Life in the event of his return.
During the Master's resurrection ceremony, one of the Broadfell Prison guards who was on Lucy's side supplied her with the Potion of Death. Lucy threw the potion on the Master, creating an explosion that destroyed the prison and killed everyone within. The Master survived, but the Potion of Death resulted in the Master being left slowly dying with a ravenous hunger, a diminishing life force and the ability to shoot bolts of energy from his hands. (TV: The End of Time) After the Master returned to Gallifrey with the Time Lords, the Time Lords repaired the damage the Potion of Death did to the Master, restoring him to normal. (TV: The Doctor Falls)