(Source: Historia Regum Britanniae, by Geoffrey of Monmouth)
I took a look back to this passage in HRB with new insights. Two things to note:
The spirits/incubi being described as "of the nature partly of men and partly of angels". This is interesting because it's implying that either these demons are a third class of beings separate from human and angel, or it's inferring the "Nephilim" tradition of half-angel/half-human hybrids, sired by fallen angels in ancient times, with the implication that these "incubi" are the disembodied spirits of said hybrids. It should be noted that the concept of angel-human hybrids (and even half-demon hybrids!) is... contested by theologians, as it is generally accepted that angels, as purely spiritual beings, cannot reproduce like animals and humans.
The abode of these entities is found in-between the Earth and the Moon. This is actually a fascinating piece of Medieval Demonology I learned about: opposite the popular idea of demons living in Hell, certain strains of Medieval thought (influenced by Greek Philosophy and certain Biblical passages) proposed that demons lived in the upper atmosphere of the Earth, referred to as the "Sub-Lunar Sphere". In Geocentricism, the Heavens are divided into seven concentric spheres, denoted by the seven classical planets - from nearest to furthest: the Moon, Mercury, Venus, the Sun, Mars, Jupiter and Saturn - with the Moon and its firmament marking the boundary between the mutable, imperfect lower world, with its constant change and decay (Our Earth) and the eternal, unchanging celestial spheres. Demons, having fallen from grace, were consigned to the lower world and tended to live high up in the clouds, occasionally descending to the ground to cause trouble. Other times, Demons were also said to inhabit the wilderness and desolate places, such as deserts.
