The name Atlas means "bearer, sustainer," and is also the name of a mythical Titan whose duty was to carry the celestial sphere.
Similarly, the first cervical vertebra in the human body is also known as the "atlas," and its Rune is: atlas. The reason is that the atlas supports the human head, a weighty part of the body, just as the Titan Atlas bears the celestial sphere on his shoulders.
Its function is to bridge the divide, elevate the bloodline, and support a non-human Rank.
It is the last and most important of the six Curse Seals of the number 666, and the requirement for the blood used is simple: the stronger the power, the better.
If it weren't for Byron's ability to manipulate blood, the Witches would need to find their most trusted person to help cut open the skin at the back of the neck and engrave the Curse Seal onto the cervical bone.
It's akin to placing one's life entirely in the hands of another.
