The New Swarm was a mysterious zerg organization.
Zerg cerebrate Kaloth acquired the corpse of former United Earth Directorate Vice Admiral Alexei Stukov, infesting and reanimating it, as an experiment in terms of infesting humans as leaders of the zerg. Stukov would later hold Dr. Emil Narud responsible for his creation. Abathur would later declare Stukov's design to be beyond that of the zerg.
The New Swarm headquartered on Braxis, allied with terrans who used the new Confederate technology, such as the Skullder Unit 001. They also captured a number of terrans and protoss, including Bud Morris.
The protoss discovered this occurrence, and so Artanis enlisted the help of Taldarin (an old dragoon) and terran Jim Raynor to put an end to the alliance. The protoss had devised a nanite serum which could reverse the infestation process.
Raynor and Taldarin fought their way past the New Swarm and terran allies, eventually meeting Stukov himself. They injected him with the nanites, removing him from the New Swarm. Stukov was bitter about this, but realized he had to flee (since the New Swarm was now turning against him). Stukov, Raynor and Taldarin fled, after which the protoss purified the planet. The renegade terrans and zerg had only been on Braxis briefly.