Devils
The Sixth Imaginary Element. Beings who cling to human wishes and try to fulfill them on a twisted manner.
The concept of devils is considered the greatest obstacle for humanity. If a god is an omniscient and omnipotent being, then a devil is unenlightened and incompetent. An abyss that can be reached but never understood.
In Fate/stay Night, only fake devils exist, and in even Fate/EXTRA, not a single phenomenon managed to create True Devil as of the current year of 2030.
Devils generally possess people, and attempt to incarnate by using human bodies, but the human mind used as their seedbed usually can't withstand the burden, severing all their connections to others and collapsing on itself.
Some human lost their humanity as result of their own acts, and feel into the devil category as result, but this is called demonization, and doesn't make True Devils.
Being possessed by a devil causes abnormalities on other components, ultimately changing even the flesh, turning them into a hideous monster. The higher ranked the devil is, the less evident its symptoms are, making the condition harder to diagnose. It only gets easy to detect it after the great disaster is already arranged.
Despite the names "devils" and "demonized humans", they function identically to meta-dimensional lifeforms, living in different planets, in different cultural spheres. That's because they no longer fit into this Universe of Awareness' definition of
human. The Outer Gods from one fictional mythology popular in the 20th century function very similarly to devils.
Sesshouin Kiara reached this territory with her transmigration in CCC's True Route, but was demoted from her divine status due to a trivial mistake.