The Breeder is a rachni queen, genetically engineered by the Reapers, which is creating rachni troops for its masters. It is encountered by Commander Shepard's team while investigating a possible rachni presence on Utukku. The Breeder only appears if the rachni queen was killed by Shepard on Noveria in 2183. The Breeder is entrenched deep within the tunnels on Utukku, guarded by Reaper technology and barriers controlled by local nodes. It speaks through the mouths of the dead krogan scout team, many of whom are suspended on the walls by the same webbing encountered on the way into the central chamber.
The Breeder is physically identical to the "true" rachni queen; however, its mind has been twisted as a result of its unnatural creation, causing it to refer to itself as a "monster" and to speak in the first-person singular rather than the plural. The Breeder is also much more insistent that it be spared, offering to spawn new children loyal to Shepard's cause in exchange for freedom. Unlike the queen, which will calmly accept its fate if left behind, the Breeder will become enraged and scream for its minions to kill Shepard should the Commander choose not to spare it.
Also, unlike the first rachni queen, the Breeder will prove untrustworthy if set free by Shepard, and result in heavy losses to the war effort.