Twenty-Seven Ancestors

The oldest 27 Dead Apostles.
However, some had already been succeed by the next generation of Dead Apostles. Nowadays it mostly referred to the distinct factions that existed among the Dead Apostles.
The Ancestors established their kingdoms consisted of Territories, Dead Apostles, and the Dead. However, some exceptional Ancestors had neither a Territory nor coven.
The Vampires who were subordinates of True Ancestors favored Territories, but those Ancestors who became Vampires as a culmination of pursuing magecraft had very little interest in such power.
The Ancestors were non-aggressive against one another. At the present, there were two main factions.
In some sense, they were an organization with power that rivaled the Church and Association.

Yoi No Myoujou: Talk.

Twenty-seven individuals who have become the apex of Dead Apostles.
A title that represents their way of existing.
There are exceptions like the “League of the Age of Gods” formed by five veterans, but each of them have a fundamentally hostile relation, and also, not all of them are enemies of humans.
Vacancies amongst the 27 has arisen over the three-thousand-year history of vampires, and excluding the six principles of Offenbaum’s principle, who was Vlov’s progenitor, the principle of “Harvest,” the principle of “Castle, therefore Kingdom,” the principle of “Sword,” and the principle of “Melting” who is sealed at the Church, along with the principle of “Four Limbs,” there are currently twenty-one confirmed remaining Apostles.
In order for the Church to seal an Ancestor, preparations are carried out for a hundred years, but it is said that there have only been two instances that they have succeeded. Taking these precedents into accounts, it is clear how irregular Vlov’s subjugation by Tohno Shiki is.

Tsukihime Material I: Tsukihime Work Glossary