Demon Eyes refer to special eyes that have abilities associated with them. How one obtains these eyes may vary, but it's possible for some to be born with them, such as someone born with the Laplace Factor, but it's more commonly known that one can be given a demon eye from Kishirika Kishirisu.
Known Demon Eyes
There are a number of Demon Eyes, and Kishirika possesses 12; however only a few are known, an aside from 6 of them, it's unknown what other eyes Kishirika possesses.
Magic Power Eyes
The most common of Demon Eyes, Magic Power eyes allow the user to view Magic Power directly. The eye appears to be a deep green color, and has a cross-like pattern, with something like 4 extra pupils appearing around the main center pupil.
When looking at something with the Identification Eyes, they will show the user details of that object. However, only within things that Kishirika knows of will be shown. Anything that she doesn't know will be shown as something unknown.
As the name suggests, X-ray Eyes allow one to see through opaque objects, including walls or clothes. However it doesn't allow one to see through people or dense magical power.
The Clairvoyant Eyes allow the user to see far away. However, the "Focus point" is difficult to control. Rather than acting as a scope, the Clairvoyant Eyes seem to change the perspective of one's vision, with the amount of mana determining the distance from the user. The original kanji (千里眼) literally means "Thousand Miles Eyes".
The Foresight Eyes allows one to see into the future, seeing possible outcomes. However, like the Clairvoyance Eyes, it's difficult to control "focus point", which in this case means how far into the future one wants to see. The further one tries to see, the more outcomes are shown to them, but these outcomes become blurred together.
The appearance of this eye has a color similar to Rudeus's natural eyes, however the pupil takes on a swirl shape, similar to a whirlpool.
These Eyes absorb magic power. However, they also absorb ones own magic, so it's disadvantageous for a magician to have the Absorption Eyes.
The All-Seeing eyes allow the user to see all things in the world, even being able to find specific people, and peer into labyrinths. However, a dense enough magical power can hinder the sight of the All-Seeing Eyes. The original kanji (万里眼) literally means "Ten-Thousand Miles Eyes", likely to indicate that these eyes are on a higher level than the Clairvoyant Eyes.
Absolute Void Eyes
The Absolute Void Demon Eyes create a barrier around a large area. It's unclear exactly how the barrier operates, but it's able to prevent a large mass of people from entering a castle.