The Asurans or Replicators were a technologically created nanite species accidentally created by the Ancients in the
Asurans originally were single microscopic nanite components, but eventually evolved to take human form and mimic their Ancient creators. A machine race they are able to display great feats of strength, probe the minds of organics, and even move through the slightest crack.
They were created to be weapons against the Wraith, thus they possess a deep aggression towards them. Included was a programming that prevented them from harming their creators. However after the meddling of Dr. McKay the Asurans were released from those restrictions and became a scourge upon the organic species of the Pegasus.
Ten thousand years ago on the planet Asuras the Ancients researched desperately for a weapon that could destroy the Wraith. One of their proposed weapons was using nanites that could attack the Wraith on a microscopic level. However their experiment had an unintended effect, the nanites suddenly began replicating and interlocking, taking the most advanced form they knew—human.
Their creations were fueled by a rage they could not understand, and begged the Ancients to remove the aggression programming, but the Lanteans refused. Seeing that they had failed to create their intended weapon the Ancients chose to destroy the nanite creations, bombarding the planet Asuras.
However the destruction of their creation was not complete. A few nanites managed to survive and reassemble themselves. After the Ancients were driven from the Pegasus Galaxy back to the Milky Way, the new race now calling themselves the Asurans reemerged and sought to complete their programming to destroy the Wraith.
The Wraith suffered from a shattering defeats, finally concluded that they could not hope to defeat the Asurans and subsequently created a shut down code that deactivated the Replicator's directive to attack them. This resulted in them returning to their homeworld where they isolated themselves to that planet alone.
There on Asuras, the Asurans continued to build their civilizations, basing their entire society on their creators. When the Atlantis Expedition rediscovered them, the Asurans welcomed them to their world. They refused to assist them in their war against the Wraith, partly due to their programming but also their contempt against humanity.
Things turned hostile when the Asurans probed the expeditions mind to locate Atlantis and its survival. The immediately dispatched a city-ship to Atlantis's location to destroy it. However they were stop by Colonel Sheppard and his team.
Much later the Asurans then began to build a fleet of ships on their homeworld Seeing this as a prelude to a invasion of Earth and the Milky Way, the Tau'ri launched an unprovoked and preemptive strike, bombing the planet. The Asurans survived and managed to rebuild thanks to their machine nature, but then sent a satellite to destroy Atlantis with a powerful beam weapon.
After Atlantis managed to escape using the hyperdrive engines the Asurans were once again attacked by the humans who stole a ZPM power source to repower Atlantis. Later McKay discovered that command code that reactivated the Asurans to attack the Wraith. In his hubris he not only forced the Asurans to fight the Wraith, but condemn the Pegasus Galaxy for the Asurans began to wipe out human populations to starve the Wraith.
The threat of the Asurans united the spacefaring races of the Pegasus, the Tau'ri, Wraith and Travelers formed a coalition to destroy them. Using a modified Replicator known as FRAN, created by Dr. McKay the Asurans were turned into a dense mass that gathered on Asuras with the weight crushing the planet and imploding it.
Asuran society appears to mimic Ancient society. They hold the Ancients in contempt as well as humanity who they see as the favored sibling that received all the parents love. While many Asurans were content to remain as they are some wished to truly become equal to their creators and Ascend.