Atomon

The Atomon was released by an experiment involving a particle accelerator on the colonial supply base Figaro Xll, by Doctor Trudo. It controlled Doctor Trudo and the base’s Dronebots to feed and grow before escaping the base. Taking on the body of a Dronebot, it swore revenge on the Time Lords who waged war against them. (DWA: The Frankenstein Particle)

The Doctor encountered the Atomon again on the planet Velgris where it was stripping the planet of its Hooji trees for its life-giving properties and threatening the native lifeforms, including the Doctor's friend Elpha. It developed a means of fusing the living matter off the Hooji tree with the metal and plastic of the Dronebots and itself. The fused Atomon teleported away but not before remarking that they wouldl meet again. (DWA: The Atomon Invasion)