Daleks

"A nightmare. It's a mutation. The Dalek race was genetically engineered. Every single emotion was removed except hate."

Pursuing the Doctor across space and time, the Daleks invaded the Earth, developed the Reality Bomb and tried to imprison the Doctor in the Pandorica. They fought the Time Lords in The Last Great Time War – a conflict so powerful and destructive that the universe was said to convulse. The Doctor first encountered the Daleks on the radiation-soaked planet of Skaro, waging war with the peaceful Thals. The Daleks were the mangled and mutated remains of the Kaled people, placed in metal war machines by the Kaleds' chief scientist Davros.

Of course, just like the Time Lords, the Daleks escaped too and the naughty little tanks teamed up with their creator Davros in a bid to use the Doctor's regenerative powers to enhance their race.