Diancecht was a Goa'uld System Lord. He lorded over the planet Herilia, on which he had brought Humans of Celtic descent from the planet Earth. They overthrew him while he was distracted with a bloody war against another System Lord, but returned to Herilia a hundred years later posing as a Herilian scientist and later bureaucrat named Sedran. He used this guise to operate in secret, and used it to develop a bio-feedback weapon which could be used against the Tok'ra and other Goa'uld, and which he experimented on the Goa'uld Arawn who was stranded on Herilia. One of his most loyal followers was a Herilian named Sulan.