The Omegaswarm is one of the UED's former 'slave broods' that has begun infesting the Nexus, assimilating the beings and essences of the realm's many worlds. The Omegaswarm sends legions of zerglings, roaches, locusts, hydralisks, banelings, ultralisks and nydus worms at you, backed up by the various infested heroes as well as Abathur's monstrosities, mines and symbiotes. Based on the loading screen art, they possess guardians and mutalisks as well.
In the events of the 'Escape from Braxis' brawl, they've consumed a significant portion of the station, forcing heroes and villains alike to work together to beat back the unending swarm and escape. Slivan the Eternal Mother is the final boss of the Escape From Braxis. In Deadman's Stand, they've moved into Deadman's Port and are actively sieging the base.
Among their ranks are various heroes and villains they have assimilated into the Swarm, including 'Aberration' Stitches, Murky (apparently an infested marine), 'Primal' Diablo, Anub'arak (as an Anubalisk), Infested Tychus, the Butcher (as a Butcherlisk) and the already Zerg Dehaka, Zagara, Kerrigan, Stukov and Abathur.