A is for... Abaddon

A vast grey horned demon that was unleashed on Cardiff after one of the Torchwood team was tricked into opening the Rift which runs through the city. This was the work of the enigmatic Bilis Manger, who the team encountered, unchanged, in both the present day and during the Second World War. Abaddon's shadow instantly killed anyone whom it touched. Many more were killed as it stomped across the city, flattening buildings. Captain Jack Harkness lured it towards him on a piece of waste ground by the docks. As he was immortal, its efforts to suck away his life-force ended with the demon's own destruction.
There is no mention of these events following Abaddon's demise, so closing the Rift may have reset things in the city.
Abaddon was one of the names which the Doctor gave to the Beast on Krop Tor. Other than their size and the horns, the creatures were not the same, physically. They may be related beings from the ancient dark times at the beginning of the Universe.

• Mentioned in both Christian and Jewish texts, Abaddon is either the name for the bottomless pit of Hell, or a demon from the pit, associated with plagues of locusts.
• Jack's climactic encounter with the demon took place on the site of what is now the Doctor Who Experience.

Appearances: Torchwood 1.13 - End of Days.