The Clade looked like a small, hand-held gun. They were intelligent, but immobile, and required hosts to do anything. They could release hundreds of minute filaments, which would dig into their host's flesh and connect the Clade to the Host's nervous system. The Clade would also attempt to mentally control its host, forcing its will on it. After the Clade took control, the host would start to decay and rot. This required the Clade to switch hosts for longer excursions.
Besides acting as a very advanced gun, the Clade had other abilities. They could cure most diseases with no damage or side effects. They could also communicate with other Clade at great distances. (PROSE: Peacemaker)
The true history of the Clade was unknown, as they removed all traces of their creators.
It was believed by the Doctor that their creators lived on the edge of the Mutter's Spiral, past the Blacklight Marches. They were attacked by an unknown force, possibly the Null, Movellans, or a splinter nest of the Racnoss. They created the Clade to defend themselves, but they were too successful and when the Clade revolted they could not be stopped. They later became mercenaries, fighting for the sake of war.
In the 1880s, a command Clade came to Earth, crashing in southern America. It was found by Alvin Godlove, who used its healing powers to work as a travelling doctor. Eventually the Clade repaired itself and wormed its way into Godlove's mind and body. Two Clades were sent to Earth to recover the commander, taking over two people, Kutter and Tangleleg, as hosts.
The Tenth Doctor and Martha, who happened to have arrived in the same area, investigated Godlove. They tracked him to an abandoned mine when they were confronted by the Kutter and Tangleleg. When the Doctor refused to help them, they injured Martha with a poison which only another Clade could heal. The Doctor and Martha eventually found Godlove, who had been completely taken over by the Clade. Godlove's body was dying, so the Clade agreed to heal Martha, provided he could use the Doctor as a host. Though the Clade was strong, the Doctor's will was stronger and he stayed in control and found the original programming. When the Clade was returned, it forced the other two Clade into a repeating programming loop, trapping them in the caves when they were collapsed. (PROSE: Peacemaker)