The Quagraemon is an outsider that is created by the Ancient artifact known as the Roark Skull. they are generally considered protectors of the Skull, and are created by forcing an Outsider into the Universe of Man, using a localized gate consuming an existing human body as Fuel. These Outsiders possess some sort of Telepathic bond with the Skull which seems to capable of directing their actions.
The Quagraemon can in turn convey other Humans into more of their kind, using themselves as a conduit for the Skulls power. With the loss of the Skull in the sinking of Atlantis, the Quagraemon have fallen into the realm of myth.