The Citadel is the name of the capital of the Devils.
Their capital exists in the easter part of the free cities and is in bad terms with the Hobgoblins, due to their aggressive expansion in the area.
Their leader is Rein Sin, the Devil Queen of the Citadel of Devils, who is also a member of The Pact of the Nine.
Despite being relativelly close to the area of the battle against the phaerimm, they did not took part in it.