Born in 1886, Golightly was given to a church-run orphanage not long after his birth, and was raised there. As an adult, he found a post as the vicar of a sleepy English village near Eddison Manor. Some time after his fortieth birthday, in 1926, his church suffered a break in, during which he discovered that he was only half-human.
He was, in fact, half-human, half-Vespiform and the illegitimate son of Lady Clemency Eddison and a Vespiform calling itself Christopher. The Vespiform side of his genetic make-up, dormant until then, asserted itself and its psychic bond to the Firestone, a gemstone in the possession of Lady Eddison. The nascent Vespiform used the Firestone to access the mind of Lady Eddison, and since she was reading a book by Agatha Christie at the time, its personality moulded itself along the lines of an Agatha Christie plot.On 8 December 1926, Golightly went to Eddison Manor, where he was born. There he murdered Professor Gerald Peach when he found out Golightly's parentage.
Later he killed Miss Chandrakala and Roger Curbishley. The Tenth Doctor and Agatha Christie deduced his part in the murders and expose his true nature. He transformed in front of the guests and chased after Agatha in a rage.
Arnold was killed, in his Vespiform form, by Donna Noble, who tossed the Firestone into Silent Pool. The psychic bond between a Vespiform and its Firestone was so strong that the Vespiform flew into the lake after it and drowned, just as his father had drowned in the 1886 monsoons.
As he died though, he had the choice of taking Agatha with him or saving her and he chose to save her for an unknown reason.