Nameless Ghouls are from the lore and stage performances of the band Ghost, also known as Ghost B.C.

Very little actual canon lore exists about the Ghouls. There is early canon lore stating that they are reanimated corpses, but this has never been touched on in canon since, and the fandom seems to prefer to interpret them as more like demons.

The one piece of consistent and fully canon information about the Ghouls is that they are the musicians in the band Ghost who play in the studio as well as tour for the stage shows. Whether it's the same Ghouls on the actual albums as play at the shows is unknown, but it is known that the band's songs are always attributed to "a Ghoul writer", who we headcanon is one or a few Ghouls chosen by whoever is currently Papa Emeritus and whose lyrics therefore tend to reflect that Papa's viewpoints.

A semi-canonical detail is that Ghouls are associated with one of the five classical elements - Water, Earth, Air, Fire, or Quintessence - due to the band's former habit of wearing the relevant alchemical symbols on the stage costumes for the bassist, drummer, keyboardist, lead guitarist, and rhythm guitarist (respectively). A Ghoul who is aligned to multiple elements at once is called a "Multi-Ghoul", and while details about what "elemental transition" entails may vary, this is the term given by the fandom for when a Ghoul decides to change their element or add an element, usually done through magic in some way.

Whether Ghouls ARE demons, per se, or if they're something else, depends on who is doing the worldbuilding, but in our version of the lore, Ghouls believe that they were the original inhabitants of Hell before demons fell from Heaven, although this happened so long ago that it is lost to recorded history and no one is sure what's true other than that, in the present day, both Ghouls and demons do live in Hell. Ghouls therefore see themselves as connected to demons due to coexisting with them for so long and often serving the same ends, but also something separate.

In our headcanon, some Ghouls live on Earth because they are either cast out of Hell (similarly to how the angels were cast out of Heaven) or through accidents involving magic, or because they are descended from Ghouls who had these experiences. Ghouls living on Earth are respected by the Church of Ghost for being so close to Hell, but also with a degree of disdain, due to now being so far from Hell.

A fanon detail holds it that Ghouls are a social species and live in packs, with children called "kits". The fandom depicts them sleeping in piles together, which is replicated with the social nesting behaviors of the Ghouls in our system. While this doesn't seem to manifest in our system, most fans hold it that different elements of Ghoul have different powers, but they all have some control over magic in some way.

Fanon also widely holds it that Ghouls are summoned by humans or people living on Earth and are therefore bound to whomever summons them, in a very transactional relationship. While we don't have a problem with this headcanon, we also don't use it, thinking it's more interesting to imagine that Ghouls are originally from Hell but now largely live on Earth, either among humans and/or in their own communities, and part of why they work with the Church of Ghost is that it's the most organized Satanic religion in their world, and they all feel ancestral ties to Hell.

While the Ghouls as performers were originally intended to be nameless and anonymous, the fandom ended up giving different musicians different nicknames that are based on the element associated in the past with their instrument. For example, the original bassist was known as Water, but since then, we have had ones known to the fandom as Mist, Dewdrop, and Rain. Earth Ghouls often have nature names, and Quintessence Ghouls, who are often headcanoned as having powers related to space or to the mind, often have names that suggest something otherworldly.

In our system, Ghouls may or may not have names associated with their elements. Sometimes it's more oblique or not in English; Dust, a Quintessence Ghoul, is called that partly because of the phrase "quintessence of dust" in Shakespeare's Hamlet, and Stein (who is part Earth Ghoul) is German with a name that means "stone" in the language.