Br'enn was a Vorta serving the Dominion in the 24th century. While some Vorta were experts in diplomacy or tactics, Br'enn was designed to be a scientist, specializing in genetics and the cloning of the Dominion's servant races. When the Cardassian Union joined the Dominion, he traveled to theToros III Shipyards to establish cloning facilities.
In late 2373 the Federation-Klingon Alliance attacked the shipyard, Br'enn and several Jem'Hadar escaped in a Dominion courier vessel. On their way to the docked ship, they encountered Seff O'Rourke and Janet Picard, who had piloted the warhead module that took out the shipyard's main reactors. A brief firefight ensued, and both Starfleet officers were captured.
He was later posted to the planet Hekiss where a Dominion Prisoner of War camp was constructed. ("Familiar Faces")
His work at the camp led to the "Alpha" Jem'Hadar breed.
After the war, he did not return to theGamma Quadrant, instead of going into hiding with a dozen of a new breed of Jem'Hadar, loyal only to him. He began cloning projects with DNA taken from inmates at Hekiss, notably Seff O'Rourke.