The servants of the dark are the main antagonist group of the entire series of Spooks books.
The dark acts as the balance, combating the soldiers of light. Therefore, there must be those who wish to fight against the soldiers of the light - and they are the servants of the dark.
These people, who commonly fight for the dark in order for self gain or preservation, are witches, boggarts and all manner of dark creatures.
All over the world, there exists hundreds, thousands even of servants, who wish to become a true member of darkness.