Demons from outside.
Sometimes they live in Vellen.
Other times they are dicks and possess people.
A Guecubu is a demon called forth from the dream realm, they were forged by other demons with the intent of discovering what life is like and how to harness that power.
Agarut, the lord over all Vellen Guecubu, has lived for many years as a dark guardian, watching the world and subtly balancing the two opposing guecubu types in Vellen society. It is passed down through Vellen tales that Agarut had come to deliver the Vellen from the Xa’tan and brought them to the Insomnium Isles to begin again. He brings darkness wherever he goes, and his voice delivers his words like commands that slowly permeate the air in rolling, dark tones.Saedokhonir, another demon that tried to impersonate Agarut when discovered to be possessing Sya’han by the Champions, but was unable to successfully do so. Sya’han was killed with Sae inside and as such Sae was presumed dead.
A characteristic unique to the Vellen, under certain conditions a Guecubu can merge with the soul of the host effectively becoming a reflection and personification of a part of the host’s mind. The Guecubu becomes a part of the host, and therefore cannot be exorcised or affected by any other effects that target demons unless the Guecubu manifests.
Guecubu grotesquely transforms the host, giving the host adaptive damage resistances as well as a new demonic form. Guecubu can also choose to remain hidden within their host, this still allows them to control the host but without transforming the host. Guecubu who remain hidden may also resist effects that target demons, such as magic circles. The transformation is not based in natural or biological processes, instead it is a manifestation of demonic processes.
Guecubu can still be exorcised, even if in advanced stages of possession.