Manannan mac Lir is a very reclusive Goa'uld System Lord who has adopted the styling of a Celtic sea god. He has control over a number of planets that he has brought ancient Celts to from the Earth, mainly from Britain, Ireland and the Isle of Man.
Manannan was a part of the brood born of the Goa'uld Queen known as Danu, along with his sister, the system lord Morrigan. Manannan has several traits that are quite atypical for Goa'uld; for one, he cares little about showmanship or military conquest. Instead, his favored means of gaining personal power are patience, negotiation, and having the proper alliances. Early in his career, he would willingly allow his siblings to claim all the glory in their battles, whilst quietly reaping the spoils.
Manannan's most important ally was a Goa'uld scientist named Arawn. Arawn gained Manannan's attention when he was in development of cybernetic Jaffa soldiers. Manannan offered Arawn a secret location to set up a research base, and the two have been allies ever since. Eventually, Manannan joined a revolt that was underway against the system lord Balor, using Arawn's advanced Jaffa to smash Balor's armies. At this time, Arawn had moved away from his research into cybernetics and had instead developed a specialized sarcophagus known as the Black Cauldron, which would turn corpses of normal Jaffa into the Cauldron-Born, utterly-obedient shock troops who were immune to both pain and fear. Claiming part of Balor's resources, Manannan petitioned, and was admitted to the ranks of the System Lords. With his new status he was granted his own queen, Morgause. He also took one of Balor's underlords, Lugh for his own.
Before long, however, Manannan's holding on Earth were put in jeopardy by an uprising lead by a human named Arthur. There are some indications that this may have been the same Arthur as in popular legend, but this is not certain. Mannan sent his queen, Morgause, to seduce and kill Arthur, but before she could execute him, she realized that her human host had somehow become pregnant. When she gave birth, she and Manannan secretly raised the boy, who was eventually implanted with one of Morgause's own larvae, Mordred.
Mordred trained extensively with Manannan's underlord, Lugh, and became a deadly warrior and a master tactician. He took care of the human rebellion on Earth that had been plaguing Manannan, killing Arthur and dismantling his forces.
- Manannan mac Lir on Wikipedia