Known in Earth mythology as the Celtic god of war, Camulus is a Goa'uld who came to power as a System Lord in the final years of his species dominance in the Milky Way Galaxy. He arranged a meeting for negotiating a treaty with Earth in the hopes of using the Ancient super-weapon to rid the Goa'uld of Baal.
Upon arrival on Earth Camulus insisted that he did not wish to involve himself in trivialities such as the names of the slaves he was negotiating with. Throughout the negotiation he enforced his species' dominance, despite the fact that he had arrived with a plea for aid.
During an impasse in the negotiations, Camulus and the others ordered the approach of a Goa'uld Ha'tak in the hopes that the ship would force Earth's hand and demonstrate the power of the super-weapon (or lack thereof). However, Baal's forces destroyed the ship.
Receiving a message that they were to be recalled because of the ship's approach, Dr. Elizabeth Weir deemed the delegates prisoners and confined them to quarters. There, Camulus requested Weir's presence and informed her that he wished for asylum on Earth. In exchange for information he would not be killed, as he had recently lost all of his forces.
Camulus remained on Earth for several weeks after the departure of the delegation until Baal requested him as trade for SG-1, who had supposedly been captured by him. Camulus, desperate, instructed General Jack O'Neill where to locate a Z.P.M. offworld. The former System Lord had previously tampered with the device and altered it so that the next time it powered a device it would destroy a solar system the size of Earth's.
Camulus, realizing Baal wished to possess him, took the Z.P.M. through the Stargate for trade, knowing Baal would destroy himself with it. However, O'Neill gave him the Proclarush Z.P.M., a drained module, instead. Camulus likely attempted to trade this to Baal. Baal, not amused, probably tortured and killed the Celtic god because of it.
A lieutenant of Camulus was Grannus.
New Order, Part 1 - Camulus arrives on Earth with Amaterasu and Yu in the hopes of trading Goa'uld hyperdrive technology for use of the Ancient super-weapon.
New Order, Part 2 - His forces annihilated, Camulus requests asylum on Earth and Dr. Weir agrees to allow him to remain.
Zero Hour - Out of options, Camulus attempts to destroy Earth with a booby-trapped Z.P.M., but is sent through the Stargate with a dud to give to Baal in the hopeful exchange for his life.
Stargate: Continuum - In an altered timeline Camulus is in the service of Baal, who has used his knowledge of the future to become the most powerful of the System Lords.