The Serpent is .... well, the Great Serpent of Myth. The Original Sin. The Father of All Dragons. Satan. Beelzabub. The Devil. You get the idea.
The Serpent has waged a campaign against the city of London and the House of Pendragon, in an attempt to create a beachhead for 'Hell on Earth'
In London, The Serpent is represented by Mr Armstrong - otherwise known as the Scourge of White Chapel. He has been killed on a number of ocassions but always manages to reappear.
In Case File; The Whitechapel Incident, the Serpent tried unsuccessfully to destablise Whitechapel
In Case File: The Black Parade, the Serpent successfully used the Serpent Sons to commit terrorist atrocities in London
In Case File: The Fall of House Pendragon, the Serpent tried to usurp House Pendragon by various means, but was thwarted by the Watch and had his temporal power neutered by the spirits of London, under the command of John Mayne.
The Serpent is no longer active in London, at the time of writing.