Arcueid Brunestud is the White Princess of the True Ancestors. The title of Brunestud was given to those True Ancestors capable of manifesting the Millennium Castle Brunestud, and has been likened to be the vampire royal family, although this is not strictly true. The True Ancestors were a race of beings copied from Type-Moon, the ultimate being of the moon, also known as the Crimson Moon. Long ago the will of the planet had no defense mechanism against humanity, nothing to keep them in check. Thus the Crimson Moon came down and offered to help the planet make a race of ultimate beings to rule over humanity, thus resulting in them being the "consciousness of the planet". True Ancestors are alike to occurrences such as earthquakes and typhoons rather than individuals as their actions, while evil and without justification, would come as naturally to her (Arcueid and other True Ancestors) as breathing. However the race was fundamentally flawed, they had a desire to drink blood that their original, the Crimson Moon, had. The True Ancestors needed to use a large portion of their power to suppress this impulse, but as there was no cure, and with passage of time the impulse grew, more and more True Ancestors fell and began drinking blood indiscriminately, while others most likely went into eternal sleep.
The True Ancestors didn't have a specific reason in creating Arcueid. They normally only mold new True Ancestors to fit specific needs. There was no need for a being as strong as her, but in the end, she originated from nothingness, simply as a product to realize the pointless goal of "the strongest True Ancestor."They used her great strength to make her into a weapon, the executioner of the fallen True Ancestors and of the most powerful Dead Apostles. According to Ciel, this happened in the 12th century AD, when the number of fallen True Ancestors was very high. She was able to perform this duty splendidly, but she was nothing more than a tool. She was taught nothing beyond what she needed to know to hunt her targets, to the extent that she even ignored the fact that she was the same kind of creature as those she was supposed to destroy, and went to sleep after every kill, with her accumulated memories erased.
As the executioner, she did not have any vampiric impulses, and the rest of the True Ancestors also believed she had none. A priest from the church seeking perfect immortality, Michael Roa Valdamjong, needed to become a Dead Apostle to continue his research as he had already reached the potential of his human body. Arcueid, ignorant of her nature as a vampire and the consequences of drinking blood, was easily tricked by Roa into drinking his and turned berserk, wiping out most of the True Ancestors, turning Roa into one of the strongest Dead Apostles and having a considerable portion of her power stolen by him, before she managed to regain her sanity long enough to chain herself to the throne and go to sleep. From then on she had to suppress her urge to drink blood like any other True Ancestors. The rest of the True Ancestors were wiped out either by Roa or the rest of the Dead Apostles, who made a game of trying to assassinate their creators.
Arcueid finally managed to destroy Roa's original body, although it took an alliance with the Burial Agency to do it. Roa was not killed however, as he had managed to master reincarnation, and constantly reincarnated into host bodies he chose before his deaths. These host bodies were chosen for a mix of social standing (rich families with influence) and the physical/magical potential of the body. After destroying Roa's original body, Arcueid went to sleep and awoke only to kill Roa's re-incarnations constantly, but was never able to destroy him and get her power back as he would simply re-incarnate each time. At the time of Tsukihime, Roa had re-incarnated 17 times, with his last re-incarnation being SHIKI Tohno.