Mnemosyne

Mnemosyne was a piece of technology stolen by a criminal. He attempted to sell it across the universe. When he came to Earth in 1903, he was killed when the humans met him. Mnemosyne hid in the London Underground, collecting psychic traces of humans who died there, turning them into what humans believed to be ghosts. Mnemosyne was defeated when Majenta Pryce stomped on her head. She was in the form of a little girl as Majenta did this, but also swore she (Mnemosyne) would get revenge on the Tenth Doctor. (DWM: Ghosts of the Northern Line)