Abaddon

Abbadon was a Demon, the son of the Beast, and was imprisoned beneath the Cardiff rift in the early days of the universe. Somehow Abbadon was able to make contact with Bilis Manger, and presumably granted him the ability to walk through time and teleport at will, and in turn did Abaddon's bidding which resulted in his release after Manger was able to manipulate the entire Torchwood 3 team to open the rift..

Once Abbadon was released and his shadow cast, only the undying Jack Harkness stood in his way. He was eventually killed by Abaddon, the Demon dying in the process due to the excess life. However, Jack was ressurected by Gwen Cooper a few days later by a kiss of life. (TW: End of Days)