Serpent of Akasha

Another name for Roa—the “extra” among the 27 Dead Apostle Ancestors, and the magus that established the art of reincarnation.
While the common goal of the Dead Apostles is to attain perfect immortality that doesn’t rely on the intake of blood, their paths toward this goal are varied.
One tried to become indestructible by turning his body into formless chaos. Another demonstrated an ineradicable existence by becoming a phenomenon.
As for Roa, he focused on the soul, rather than the body, mastering magecraft that allowed him to reincarnate and assimilate into another individual upon the demise of his physical form.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary