Crimson Red Vermillion

The name for when those with inhuman heritage have drawn out their dormant blood to the maximum possible extent. Once they reach this state, they are enveloped in a mirage-like haze and can never return to being human.
The name Crimson Red Vermilion is unique to the families ruled over by the Tohno household. Elsewhere, it is known as Ancestral Return.

Tsukihime Data Collection: Tsukihime Dictionary
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This is the name of “borderline demon beings” which were born of the Tohno family line with its age-old demon bloodline. Also known as “Ancestral-Return.” Once a member of the family is overwhelmed by their demon blood, they can never return to being human again. In Tsukihime and Kagetsu Tohya, the Kishima branch family (of the Tohya family), have inherited especially thick demon blood. The label Crimson Red Vermillion is used for those of mixed-blood descent who drew out their non-human blood to the max.

MB Official Strategy Guide: Melty Blood Glossary
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The name for when those with inhuman heritage have drawn out their dormant blood to the maximum possible extent. Once they reach this state, they are enveloped in a mirage-like haze and can never return to being human.
The name Crimson Red Vermilion is unique to the families ruled over by the Tohno household. Elsewhere, it is known as Ancestral Return.
As one with particularly tainted blood, the possibility that Akiha will become a Crimson Red Vermilion is high. In Tsukihime, when her hair turned deep crimson, she was on the verge of this, though somehow she managed to bring her human and oni blood under control. Of course, rather than returning to how she was before, it's more accurate to say she switched to her “human” self while remaining close to the Crimson Red Vermilion state.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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The name for when those with inhuman heritage have drawn out their sleeping blood to the maximum possible extent.
The name Crimson Red Vermillion is unique to the families ruled over by the Tohno household. Elsewhere, it is known as Ancestral Return.
As one with particularly tainted blood, the possibility that Akiha will become a Crimson Red Vermillion is high.
You can catch a glimpse of this during Tohno Akiha and Kishima Kouma's Arc Drives. Also, though Kouma is the closest to attaining it at the current stage, Akiha is actually the one with more aptitude for it.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary