Diancecht was a Goa'uld System Lord. He sowed a group of people by the Celts descent. Herilians threw out his regime in a bloody insurgency centuries ago, taking advantage of the fact that resources were spread thin Diancecht at the time because of a particularly bloody war with another System Lord. System Lord was not to conquer the planet by force. Instead, he worked behind the scenes slowly shaping society in an experiment to see if a world can be more useful to him if the Europeans did not know he was really in charge.
For years it seemed this experiment a complete failure, but Diancecht recently stumbled upon a development Herilian technology that may be useful in the war between the System Lords and the Tok'ra - a survey on human bio-feedback. Diancecht recognized immediately combat applications of this technology, previously ignored by Goa'uld since generally helped people.He was later killed by Tauri agents. (RPG: "The Herilian Agenda")