Zarak

Zarak was a member of the Xeraphin species. After the Vardon-Kosnax War, he took control of the Xeraphin Senate by imprisoning its leaders. At the direction of the First Doctor, he and the rest of the Xeraphin merged into a bioplasmic organism in order to escape the effects of radiation poisoning. (PROSE: The Vardon Horse)

After 's arrival on prehistoric Earth, Zarak was a manifestation of the evil side of the Xeraphin who wished to help gain control of the Xeraphin and seize power. (TV: Time-Flight)