Tasdevin

 was a scientist who hoped to create a perpetual motion machine to solve the energy needs of his homeworld.

Tasdevin worked on the Schrödinger Institute. He tried to stabilise a portion of space-time, allowing it to recycle constantly, so that he would never age. His experiment caused a temporal echo in which the Doctor and Guinevere Winchester were trapped. Before the Doctor stopped the cycle, he and Guin witnessed an echo of Tasdevin start his machine and threaten fellow scientist Groote with a knife. (PROSE: Revenants)