The Quantum Assassin

 was after Drax who had some of his stuff. He lured the Eighth Doctor to help get the information about the money that Drax has stolen from him. He posed as Drax to do this. When the Doctor realised where the Money was, he dropped the act. However, the Fourth, Seventh and Tenth incarnations of Drax trapped him in a Schroedinger Cage, and dropped him in the Black Hole of Tartarus. This was because the assassin lived in multiple timelines and this was the only way to trap him. (AUDIO: Here Lies Drax)