Nature spirits in the form of vampires created as a countermeasure against the threat of humanity corrupting the environment of Earth.
While appearing as vampires, True Ancestors are actually spirits within nature given life by their creator, Brunestud of the Crimson Moon. Created as an agreement between Brunestud and the incarnation of the Earth, Gaia, True Ancestors were created as a countermeasure to combat the threat of humanity should its very existence threaten the planet's stability. As nature spirits, True Ancestors have the ability to use Marble Phantasm, the ability to will one's control of nature upon the Earth and control probability, which can vary from manipulating the weather and environment to controlling animals.
Like vampires, True Ancestors experience bloodlust, though they don't require blood for sustenance as the desire is more psychological and a major flaw inherited from Brunestud. Because of this bloodlust, True Ancestors were forced to use much of the power of their Marble Phantasms to keep the urge in check. Unfortunately, this isn't something that can be totally preventable as True Ancestors eventually give into their bloodlust and became Demon Lords, those who willingly gave into bloodlust and can now exert full use of their Marble Phantasm. The existence of Demon Lords would lead to the creation of Dead Apostles, humans turned into vampires by either a Demon Lord, magic or another Dead Apostle.
To counteract the threat of Demon Lords and Dead Apostles, Brunestud would create Arcueid as their executioner. She was an almost perfect copy of her creator in power that Arcueid didn't carry the bloodlust found in other True Ancestors and could summon the Millennium Castle Brunestud, her creator's lair and a piece of his Reality Marble, with her Marble Phantasm. Through a combination of Arcueid having to kill True Ancestors who gave into bloodlust and Roa killing them off to get rid of those who created him, Arcueid is now the last living True Ancestor.