Captain Stoker was a first-generation Ph'Sor made by combining the DNA of Tzun and cloned human tissue.
The creation of these first-generation Ph'Sor was flawed, causing them to have uncontrolled emotional outbursts. Stoker was convinced by the Tremas Master that the Tzun were going to eliminate him and his fellow Ph'Sor, so he allied himself with the Master. When he discovered that the Tzun were in fact going to correct the genetic error, he had already outlived his usefulness and the Master killed him. (PROSE: First Frontier)