True Demon

Even in the EXTRA world, things which fall under the category of demons are treated as the greatest obstacle to mankind’s continued existence.
Humans think of God as a being who is all-knowing and all-powerful, but a demon is an existence that cannot be comprehended through human intelligence or knowledge.
The concept of what a demon is lay within reach, but it is part of an abyss that is impossible to truly comprehend.
In Fate/stay night, the only demons that appear are fakes; humans could never match up to a true flesh-and-blood incarnated demon.
(There’s only one record of a flesh-and-blood incarnated demon being dispelled, but that’s another story).
Demons are normally said to be beings which possess humans and try to make a human body their own, but the mind of any human they posses to make their own is unable to bear the great burden, and so the body and demon end up disintegrating into pieces which spread evil power into the surroundings.
This definition of demon also includes humans who have lost their humanity due to their actions and as a result fall into the same category as demons. However a human who has transformed into a demon is not a True Demon.
In the EXTRA world, in present day 2030, a True Demon has never been successfully born into the world, this includes through demons possessing humans and humans transforming into demons through their actions.
We talk about demons and humans transformed into demons, but really as far as we are concerned demons are no different than a higher dimensional beings from a different planet, different civilization. No matter where you go in the known universe, they would never be recognized as “human.”
The existence of a True Demon is close to that of the evil gods from a certain creation mythology that was popular in the twentieth century.
In the True End Route of CCC, a certain reincarnated person who has essentially attained True Demon level loses their seat as a god due to a clumsy mistake.

Fate/EXTRA Material: Encyclopedia of Fate/EXTRA