Mystic Eyes

Typically, single-action spell possessed by magi.
They are created by changing the eyeballs, which by nature are passive receptors of information from the external world, into tools for actively influencing the external world.
People within the user's field of vision will have spell cast on them without question, and should they happen to make direct eye contact the effect will be dramatically increased.
In short, they are a fearsome magecraft trait—not only are they something that must not be seen, but simply being seen by them means you have fallen into your opponent's trap.
Because of their concealability and capability, they are considered the mark of a first-class magus.
Though, artificial mystic eyes are limited to effects on the level of charm or suggestion.
The most powerful mystic eyes are those which are possessed from birth.
Things like Rider's mystic eyes of petrification, or mystic eyes that can see the death of things, are “psychic power” that cannot be reenacted even with magecraft.

Fate/side material: Fate Encyclopedia
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Typically, single-action spells possessed by magi.
They are created by changing the eyeballs, which by nature are passive receptors of information from the external world, into tools for actively influencing the external world. Humans within the user's field of vision will have the spell cast on them without question, and should they happen to make direct eye contact the effect will be dramatically increased. In short, they are a fearsome magecraft trait—not only are they something that must not be seen, but simply being seen by them means you have fallen into your opponent's trap. Because of their concealability and capability, they are considered the mark of a first-class magus. Though, artificial mystic eyes are limited to effects on the level of charm or suggestion.
The most powerful mystic eyes are those which are possessed from birth. Things like Shiki's mystic eyes that can see the death of things, or mystic eyes that can turn someone to stone with a glance, are “psychic abilities” that cannot be reenacted even with magecraft.
Mystic eyes can be ranked by color. Normal mystic eyes shine red or green. Strong ones glow a luminous gold. And eyes that fall in the realm of gods glimmer with multiple colors, like jewels or rainbows.
By the way, though Akiha's “Origami” bears a striking resemblance to mystic eyes, it is actually a kind of curse. And despite Shiki's eyes being called the “mystic eyes” of death perception, strictly speaking they are a psychic ability—specifically, the “pure eyes” possessed by saints and holy men.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised