The name of the time-traveler, Bilis, is an anagram of "iblis", the primary name of the devil in Islam.
The name "Abaddon" was first mentioned in the Doctor Who two-part episode "The Impossible Planet/The Satan Pit" as one of the names for the Devil.
This was shot without any of the cast or crew knowing whether or not the series was going to be renewed.
Up until this episode, of all the Torchwood members, only Gwen was privy to the fact that Captain Jack cannot die.
The cut that appears on Tosh's mother's head and on other people are all the same cut that appears on the child's hand in the Doctor Who episode 'The Empty Child'.