Prior in the service of the Ori, who attempted to establish a beachhead in the Milky Way Galaxy by helping the Ori create a Supergate. After traveling to the planet Kallana and dispatching its Jaffa inhabitants, he surrounded the area immediately surrounding the Stargate with a blue energy field ... and began chanting.
The Prior knowingly and willingly gave his life for his goal, anticipating that enemies of the Ori would fire upon his position with a high-yield weapon. The field around the gate only absorbed the energy, eventually growing large enough to surround the planet and convert it into a singularity to power the planned Supergate.
Nothing is known about the man before he became a Prior. He is presumably a native of the Ori's home galaxy.
Beachhead - A Prior of the Ori sacrifices his life to turn the planet Kallana into the power source for a Supergate.