Research by Toshiko.
Abaddon is a Hebrew word, orignally meaning “The Realm of the Dead” in the Old Testament. By the time of the New Testament, Abaddon is embodied as The Destroyer – sweeping across the Earth like a plague of locusts at the end of the world.
Beyond that, we’re pretty much stuck with what Jack told us – that, at some point, a creature called Abaddon was entombed underneath Cardiff, with the Rift used to somehow keep it there.
Which begs the questions – Who would have the power to do that? Where are they now? Were there other beings like Abaddon? Are they also entombed underneath planets across the universe?
These are not good questions.