Abaddon

Abaddon was a Demon and the son of the Beast. He was imprisoned beneath the Cardiff rift on Earth.

Powers
Abbadon was known as "the Devourer" for his ability to devour life. His very shadow would kill humans, but his abilities were intended to feed on the Dark. (TV: End of Days, PROSE: The Twilight Streets)

Biography
Abaddon was an ally of the Light, who fought against Pwccm and the Dark. They fought together for centuries, travelling the dimensions until they were caught up in the Cardiff rift. (PROSE: The Twilight Streets)

A 1827, archaeological expedition excavating the ruins of Babylon found a stone tablet depicting a Demon standing above a city. (WEB: torchwood.org.uk)

In 1876, the Rift was opened in Tretarri, where Abaddon and Pwccm fought. Though Abaddon appeared to be defeated, he had in reality faked the loss so that he could gain power. He also set his servant, Bilis Manger, to have the event recorded in a special diary, which would be used later to free him. (PROSE: The Twilight Streets)

Later, Bilis set in motion a series of events designed to manipulate the Torchwood 3 team into using the Rift manipulator to open the rift and release Abaddon. 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. (TV: End of Days)

Abbadon was known of by the Xylok Mr Smith. (TV: Revenge of the Slitheen)

Following the defeat of Abaddon, the Light was greatly weakened and was forced to fight against the Dark and Pwccm on their own. After Torchwood sealed the Dark away and sent the Light back into the Rift to protect it, Abaddon's ashes appeared out of the ground and were recovered by Bilis Manger to be buried outside of Britain. (PROSE: The Twilight Streets)