The Ph'Sor were one of the Tzun sub-species.
After fighting the Darklings, the Tzun were left with corrupted DNA. They developed genetic engineering to an extremely advanced level. This led them to the creation of two sub-species. One was the S'Raph, who could withstand high pressures and see into the infrared and ultraviolet. The other was the Ph'Sor, which were made by combining DNA from the Tzun and the races they conquered. This allowed them to infiltrate other planets for invasion, and enable them to adapt to more varied environments.
In 1957, the Tzun created Ph'sor from cloneed human tissue. The first generation had emotional control problems. They used them to lay the groundwork for their takeover of Earth, having them claim to be benevolent aliens from Venus who were going to bring universal peace to the world. The leader of these Ph'sor was Xeno. (PROSE: First Frontier)