Asurans

The Asurans are a fictional race in the science fiction series Stargate Atlantis that bear many similarities to the human-form Replicators of Stargate SG-1. These similarities have led the Asurans to simply be called "Replicators" by other characters on the show. They are first introduced in the season 3 episode "Progeny".

The self-replicating nanites that would eventually become the Asurans were created by the Ancients on the world of Asuras (M7R-227), as a weapon against the Wraith. As they grew in numbers and sophistication, they assumed the form of their creators. The Ancients programmed the Asurans with an aggression rivaling that of the Wraith, but when they determined that the Asurans would never become the weapons they desired, they destroyed their experiment and deleted all references to it in their database. A few nanites survived, however, and rebuilt their civilization on a grand scale. As per their programming, the Asurans attacked the Wraith, who were able to deactivate their attack command using a computer virus and cause them to withdraw to their homeworld. A minority of Asurans, led by Niam, are interested in Ascension.

The existence of a second advanced enemy in the Pegasus galaxy is first hinted at in the season 1 episode "Hot Zone", in which the Atlantis Expedition encounters a nanotechnological pathogen that infects and kills several personnel, but did not affect those with the ATA gene. In season 3, the Atlantis team makes first contact with the Asurans. Upon learning that they have occupied Atlantis and are the Ancients' favored "second evolution" from Earth, the Asurans launch an attack on Atlantis. The Atlantis team foils the attack after Dr. Rodney McKay accesses their base code. However, his tampering also allows the Asurans to override the programming that prevents them from harming the Ancients. This becomes evident when the Asurans launch a second attack on Atlantis in "The Return", which at that time had been reclaimed by Ancients. The Asuran strike force occupies Atlantis until they are defeated by the former Atlantis team.

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Wikipedia