Dead Apostle

The bloodsuckers that make up the vast majority of the subcategory of bloodsuckers known as vampires.
…That was a confusing sentence!
Originally they were meals for the True Ancestors, who would prepare humans in advance for the times when their vampiric impulse became impossible to control. Of course, to keep them from escaping, the True Ancestors sucked their blood and brought them under their control.
These humans who had their blood sucked by the True Ancestors in turn began to suck blood themselves. Though, in their case it was mostly as a means of survival. After all, it stands to reason that the apostles serving the extremely long-lived True Ancestors must be possessed of similarly long lives as well.
However. At some point, after sucking the blood of others for such a long time, they began to feel a sense of superiority from the very act of sucking blood itself. They strengthened their abilities, their will, and eventually escaped from the control of the True Ancestors into the world of man, where they repeated the act of drinking blood over and over again to maintain their existences.
This is the origin of the bloodsuckers formed from humans, the Dead Apostles.
The Dead Apostles that escaped the True Ancestors the earliest are known as the 27 Ancestors. Though some of those 27 have been replaced over the generations, there are also those that still reign as Ancestors even now.
In the story, Sacchin was lucky enough to have a body blessed with excellent potential, and completed her rebirth into a bloodsucker in under half a day. It wouldn't be wrong to say her potential as a vampire is close to that of the 27 Dead Apostle Ancestors of old.

Tsukihime Data Collection: Tsukihime Dictionary
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To put it simply, Dead Apostles are your average/normal vampires in the Tsukihime lore. You can find out all the juicy details about them by reading through Tsukihime. The first 27 Dead Apostles which were created by the True Ancestors are called “The Twenty-seven Dead Apostle Ancestors” and are the strongest of the Dead Apostles. However, a number of them have already been taken down by the Church. The 10th Dead Apostle Ancestor is Nrvnqsr Chaos, and Roa who is ranked outside of the list due to his uniqueness, were both taken out by Shiki and company in Tsukihime. In Melty Blood the concept of Dead Apostles doesn't really exist.

MB Official Strategy Guide: Melty Blood Glossary
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The bloodsuckers that make up the vast majority of the subcategory of bloodsuckers known as vampires.
Originally they were meals for the True Ancestors, who would prepare humans in advance for the times when their vampiric impulse became impossible to control. Of course, to keep them from escaping, the True Ancestors sucked their blood and brought them under their control.
These humans who had their blood sucked by the True Ancestors in turn became bloodsuckers themselves. Though, in their case it was mostly as a means of survival. After all, it stands to reason that the apostles serving the extremely long-lived True Ancestors must be possessed of similarly long lives as well.
However. At some point, after sucking the blood of others for such a long time, they began to feel a sense of superiority from the very act of sucking blood itself. They strengthened their abilities, their will, and eventually escaped from the control of the True Ancestors into the world of man, where they repeated the act of drinking blood over and over again to maintain their existences.
This is the origin of the bloodsuckers formed from humans, the Dead Apostles.
The Dead Apostles that escaped the True Ancestors the earliest are known as the 27 Ancestors. Though some of those 27 have been replaced over the generations, there are also those that still reign as Ancestors even now.
In the main story of Tsukihime, not only was Yumizuka Satsuki's transformation into a vampire caused by having her blood sucked by Roa, but she was also lucky (?) enough to have a body blessed with excellent potential, allowing her to complete her rebirth into a bloodsucker in under half a day. It wouldn't be wrong to say her potential as a vampire is close to that of the 27 Dead Apostle Ancestors of old.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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Vampires. Basically, humans that have by some means or another been transformed into bloodsuckers.
Originally they were emergency meals prepared by the True Ancestors, to use when they became unable to control their vampiric impulse. In a sense, you could say they were a type of pain-killer.
Humans who had their blood sucked by the True Ancestors would in turn be transfigured into similar bloodsuckers themselves, and forced to spend their now immortal lives serving as the True Ancestors' emergency rations. Without a doubt, they were a “company of the dead.”
However, whereas the True Ancestors' desire to drink blood is purely mental, the Dead Apostles need to drink blood to maintain their bodies. Or to put it another way, for the True Ancestors it is merely entertainment, while for the Dead Apostles it is a fundamental aspect of their survival similar to breathing.
At some point, after sucking the blood of others for such a long time, the Dead Apostles began to feel a sense of superiority from the very act of sucking blood itself. They strengthened their abilities, their will, and eventually escaped from the control of the True Ancestors into the world of man, where they repeated the act of drinking blood over and over again to maintain their existences.
This is the origin of the bloodsuckers formed from humans, the Dead Apostles.
The Dead Apostles that escaped the True Ancestors the earliest are known as the 27 Ancestors. Though some of those 27 have been replaced over the generations, there are also those that still reign as Ancestors even now.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary
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Creatures that made up the majority of the Blood-Suckers. Considered to be the prototypical Vampires.
They were originally the emergency rations of the True Ancestors. The Dead Apostles served as living blood bags when True Ancestors could not curb their vampiric impulses.
These creatures whose blood was drained by True Ancestors and bound as servants were called Dead Apostles.
Within the ranks of Dead Apostles, there were those who were unrelated to True Ancestors. They were the ones who acquired immortality (as Vampires) at the end of their quest for magic. They also joined the secret society of Dead Apostles. Although their origins differ, they were still considered Blood Suckers.

Character Material: Prelude Explanation