From the writings of Motoman Nine Eyes
The specters are lesser demons that came into this world along with Belzaor. They seem limitless in numbers, records during the Terror spoke of them like a tide rushing over everything, but they have weaknesses, the foremost of which is that they seem only semi-sentient. They are attracted to light, noise, like any predator, but otherwise they mill and roam rather aimelessly, in small numbers, congregating only where they are drawn by dark energies.
They do not leave the Blightlands, that is unless they are being lead by a marshal or war leader, one of their kind which are very much sentient, powerful, capable of magic, and thankfully rare.
The marshals themselves have a singular leader, the Lord Marshal, Phanavaom. He has a strategic mind, is ruthless and endlessly cruel, but the worst of it seems to be he cannot be destroyed. Whenever he is felled he rises anew, emerging from a lesser specter, as from a chrysalis.