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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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
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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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Vampires that possess a nearly immortal body. They were formerly human, but they transformed after having their blood sucked by a True Ancestor or another dead apostle. A dead apostle’s body is constantly decaying. Sucking human blood prevents this. Shirley becoming a dead apostle on Alimago Island is quite irregular because it is the result of Norikata’s research. However, after her transformation into a Dead Apostle, she is unable to restraint her desire to suck blood, so Norikata deems it a failure.

Fate/Zero Animation Guide II: Glossary of the holy grail war II
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Refers to those that became vampires after having their blood sucked by either a True Ancestor or another Dead Apostle. Makes up a large portion of the beings referred to as vampires. Their bodies are nearly immortal; however, they can’t deal with being exposed to the sun. …Although related [by name and in concept], the zenith of the Dead Apostle hierarchy known as the Twenty-seven Dead Apostle Ancestors does not exist in any world in which heroic spirits can be summoned.

MB Back Alley Alliance Nightmare: Encyclopedia of Rojiura Nightmare
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Something much like what the public at large imagines a vampire being like.
Immortal, weak to sunlight, sucks human blood, transforms into beasts or mist, etc…
They’re called vampires in the game as to make things a bit easier to understand.
*However, the creature known as the vampire started being talked about in the 1900s. In real folklore, they’re actually the monster with the lowest pedigree. In contrast, werewolves, who are often seen as their henchmen, are monsters passed down from days of old—in fact, parts of what the vampire was modeled after were lycanthrope characteristics.
That’s why in the world of Tsukihime, they are more properly treated as bloodsuckers—the general term for things that suck blood from their own kind.

Dead Apostles are—just as one would imagine from their name—dead. They are divided into those that have been transformed into one by some kind of magecraft, and those that have had their blood sucked by a True Ancestor, and become its puppet.
Unlike True Ancestors, both of these kinds have to suck human blood in order to survive. Blood is considered a necessity for them to live.
They’re also weak to sunlight, and as they’re unable to completely rid themselves of the shared awareness of being human, they are defeated when faced with religious cleansing rites.

Katatuki Kouhon: World & Tsukihime Character Setting