The Zentra were much taller than Time Lords and could easily pick the Eleventh Doctor up with one hand. They had purple skin. They could fire bursts of energy from their hands. (COMIC: The Zentrabot Invasion)
The Zentra used Zentrabots as maintenance droids to maintain their rusty spacecraft. When they came to Earth, they disguised Zentrabots as the Christmas "toy craze". When the Zentrabots were underneath children's Christmas trees, the Zentra sent an activation signal to kidnap a hundred thousand children, and demanded to one of Earth's governments that they would hand over all its weapons or the children would die.
A Zentra guard discovered the Eleventh Doctor and Danny reprogramming the Zentrabots on board the Zentra mothership. The Doctor warned their captain, Karvox, that he would send the signal that would make the reprogrammed Zentrabots attack the guards. Karvox ignored him and the Doctor successfully sent his signal.