True Ancestor

A singular existence amongst the bloodsuckers. Characteristically close to elementals.
They were created by nature, which had no tangible method of self-defense toward humans. Therefore, they are extensions of nature.
“If mankind is to be ruled, let man be the model.” Under this premise, they were created with a mind and body structure similar to humans.
However, there was a birth defect. The True Ancestors bore a flaw, the vampiric impulse.
Each and every True Ancestor is burdened with the desire to drink the blood of the very humans they were meant to rule. And, in order to suppress that impulse, they consume their own willpower.
Those that can change the state of the world merely by thinking are instead forced to use almost all of those “thoughts” to restrain themselves.
But, everything has a limit.
Unable to resolve the problem at its root, the vampiric impulse accumulates, until finally it becomes impossible for them to control with their own power alone.
True Ancestors that have become unable to control their desires fall into an eternal slumber. It could be said that this is the lifespan of those who have no end.
Also, there were those among them that succumbed to their desires and began indiscriminately drinking human blood. Known as Fallen Ancestors, they are powerful to the point that it is said to be completely impossible for humans to oppose them.
Though the True Ancestors possess godlike abilities, the fact that human ideas had no influence on their creation means they are not a type of Divine Spirit. They are an existence desired by the world, but not by humans. Because of this, they have gradually gone into hiding and dwindled in numbers.
Though they have definite form, they can also be considered a kind of Counter Force. …If you want to know what a Counter Force is, please refer to Kara no Kyoukai.

Tsukihime Data Collection: Tsukihime Dictionary
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A type of vampire, but really they are closer to being spirits. Of the available characters Arcueid is a True Ancestor. True Ancestors serve as a defense mechanism for nature [Gaia, the earth], since nature possesses no natural defense mechanism capable of stopping the human race. The True Ancestors have a fatal flaw in that they succumb to vampiric blood sucking impulses, and True Ancestors who fail to resist the urge to suck blood are labeled as Fallen True Ancestors, and hunted by Arcueid. Also, a human who has their blood sucked by a True Ancestor becomes a Dead Apostle.

MB Official Strategy Guide: Melty Blood Glossary
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A singular existence amongst the bloodsuckers. Characteristically close to elementals.
They were created by the planet, which had no tangible method of self-defense against humans. Because of this, it could be said that they are nature's sense of touch. “If mankind is to be ruled, let man be the model.” Under this premise, they were created with a mind and body structure similar to humans. However, there was a birth defect. The True Ancestors bore a flaw, the vampiric impulse. Probably, it's because the original that was used as a reference had contained that flaw as well.
Each and every True Ancestor is burdened with the desire to drink the blood of the very humans they were meant to rule. And, in order to suppress that impulse, they consume their own willpower. Those that can change the state of the world merely be thinking are instead forced to use almost all of those “thoughts” to restrain themselves.
But, everything has a limit. Unable to resolve the problem at its root, the vampiric impulse accumulates, until finally it becomes impossible for them to control with their power alone. True Ancestors that have become unable to control their desires fall into an eternal slumber. It could be said that this is the lifespan of those who have no end. Also, there were those among them that succumbed to their desires and began indiscriminately drinking human blood. Known as Fallen Ancestors, they are powerful to the point that it is said to be completely impossible for humans to oppose them.
Their creation was not influenced by human ideas, so they are not a type of Divine Spirit. They are an existence desired by the world, but not by humans. Because of this, they have gradually gone into hiding and dwindled in numbers. Though they have definite form, they can also be considered a kind of Counter Force.
…If you want to know what a Counter Force is, please refer to Kara no Kyoukai.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
§

A singular existence amongst the bloodsuckers. Characteristically close to elementals.
They were created by the planet, which had no tangible method of self-defense against humans. Because of this, it could be said that they are nature's sense of touch. However, there was a birth defect. The True Ancestors bore a flaw, the vampiric impulse.
Each and every True Ancestor is burdened with the desire to drink the blood of the very humans they were meant to rule. And, in order to suppress that impulse, they consume their own willpower. Those that can change the state of the world merely be thinking are instead forced to use almost all of those “thoughts” to restrain themselves.
But, everything has a limit. Unable to resolve the problem at its root, the vampiric impulse accumulates, until finally it becomes impossible for them to control with their power alone. True Ancestors that have become unable to control their desires fall into an eternal slumber. It could be said that this is the lifespan of those who have no end.
Year by year their numbers dwindled, until by the time Arcueid was born in the 12th Century, only a hundred individuals remained.
Also, there were those among them that succumbed to their desires and began indiscriminately drinking human blood. Known as Fallen Ancestors (Demon Lords), they are powerful to the point that it is said to be completely impossible for humans to oppose them.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary