Arawn first entered the service of Manannan after a military defeat at the hands of a minor Goa'uld named Gwydion. Gwydion had stolen several artifacts from Arawn, and in response, Arawn sent some of his highly advanced cybernetic Jaffa to recover them. Arawn's forces were defeated only because they were hopelessly outnumbered, and Manannan was impressed by the tenacity of the enhanced Jaffa. Mananna offered Arawn his own world, Annwn, as a scientific base, and Arawn accepted. He has provided Manannan with technology and troops ever since.
Eventually, Arawn moved away from his research into cybernetics, and instead developed a specialized sarcophagus known as the Black Cauldron, which would turn normal Jaffa into utterly-obedient shock troops who were immune to both pain and fear. These "cauldron-born" Jaffa would become the backbone of Manannan's forces.
Despite being a gifted scientist by Goa'uld standards, Arawn craves power and wealth as much as any System Lord. Secretly, he acknowledges that he is not cut out to be a powerful leader among the Goa'uld, but in public he makes it a point to act as sinister, petty, and evil as any Goa'uld. He was later captured while marooned trying to take over the planet Herilia, which was formerly ruled by the Goa'uld Diancecht. This Goa'uld was ousted by the Herilian people, but was secretly ruling in secret. The deception allowed him to test a new bio-feedback device developed by the Herilians on Arawn. (RPG: "The Herilian Agenda", "Living Gods: Stargate System Lords")