The Asurans are a techno-organic life-form created by the Ancients.
History
During the war with the Wraith, the Ancients sought to create a weapon which would attack Wraith DNA. Turning to nanite technology, the Ancients built nanites which would allow for organic assimilation and replication, encoded with an aggression core that surpassed even that of the Wraith. Without warning, the nanites suddenly began replicating and interlocking, taking the most advanced form they knew—Ancients. But the aggression codes remained. Fueled by a rage they could not understand, the machines begged the Ancients to remove the aggression programming, but the Ancients refused, for they had insured base codes that prevented the Asurans from harming them, and they were desparate to find a weapon to destroy the Wraith. When they concluded that the Asurans would not be the weapon they sought, the Ancient Council sent a fleet of Aurora class warship to Asuras, wiping out the laboratory and the nanites, and deleted all references to the project from their database, save for Asuras' Stargate address. However, some of the nanites survived, and began to replicate again. (SGA: "Progeny")
Ten thousand years later, the Asurans had achieved much: They had built their own city-ship, as well as sorrounding structures, and were able to manufacture Zero Point Modules. Most of the Asurans, such as Oberoth, were content to live as they had, but some, like Niam, wanted to ascend. When they learned of the Atlantis Expedition, the Asurans welcomed them, claiming that they were Ancients who had quarreled with the Atlanteans over the war with the Wraith. After the learning that they needed Zero Point Modules, they probed the minds of Atlantis 1 and Elizabeth Weir, and learned that Atlantis was intact. Intending to destroy Atlantis out of retribution, they took the city-ship to Atlantica. However, Rodney McKay, with the help of Niam, tried to remove the aggression core programs. Instead, McKay froze the Asurans, and rigged the city-ship to detonate. Taking Niam with them, Atlantis 1 escaped, but were soon forced to leave Niam in space after the other Asurans reset his programming and attacked he Weir. (SGA: "Progeny")
Later, after Atlantis had been reclaimed by live Atlanteans, the Asurans attacked the city. Finding a way to overcome the code which prevented them from harming the Ancients, they attacked the city, occupying it and taking Major General Jack O'Neill and Richard Woolsey hostage. John Sheppard, Weir, McKay, Ronon Dex, Carson Beckett, and Teyla Emmagan hatched a daring plot to resuce O'Neill, Woolsey, and Atlantis. They initially attempted to use Niam to upload a virus into the Asurans which would freeze them for seven hours, allowing them to attack the Asurans. However, Niam reactivated, forcing them to kill him. As a new plan, they modified the shield emiters to release a massive replicator disruptor wave, destroying the Asurans who invaded the city. (SGA: "The Return")