Abaddon was a Demon and the son of the Beast.
imprisoned beneath the Rift possibly by the disciples of the light as they imprisoned his father on the planet Krop Tor.
A 1827 archaelogical expedition excavating the ruins of Babylon and found a stone tablet depicting a great demon standing above a city.
Bilis Manger, considering himself Abbadon's servant, set in motion of series of events designed to manipulate the entire Torchwood 3 team into using a technological device to open the Rift and so release the demon. This done, the Rift released Abaddon and he appeared. Abaddon seems to share none of the intelligence of his father and was not shown to speak or plan. There is still the possibility that he was the one who created the plot, rather than Bilis Manger himself.
Torchwood:End of Days(2006): As Bilis Manger gets his way when the Torchwood team open the Rift. Abaddon, son of the Beast rises from the rift....