Quoting Christian Schlichting
There are different cults for each religion and also for demon followers e.g. the cult, the dark whorship of the high lords of the new world of the unholy circle of demons, chaos and the last days (see wikilegonia:religion).

demons and demongods are mostly formless, bound on special places or banned from our dimension, so yes, you can make them how you want (black, monstrous, elemental...)