Only he who holds the Blood-Sin can succeed to the Dark city. The would-be successor must cede his phantom soul if he is to accept his prize - control of the Dark.
Whoever bears the Blood-Sin upon their back can control the Dark and the city of Leá Monde. It was first borne by the Kildean priestess, Müllenkamp, and those who followed her cult. After the influence of the cult waned and the Iocus priesthood dominated the region, the Blood-Sin symbol was adopted and inverted to be known as the "Rood of Iocus" (ホーリーウィン, Hōrīuin?). This symbolizes their aversion of the Darkness the cult had worshiped. The priesthood had since labeled the Blood-Sin symbol as the "Rood Inverse."
The lineage of Müllenkamp survived, and the Blood-Sin passed through the family of Duke Bardorba. Sydney Losstarot, the Duke's son, holds the Blood-Sin, his goal to pass it off to another who will not abuse its Dark powers. Both Jan Rosencrantz and Romeo Guildenstern desire the power of the Dark, and search for "the Key" not knowing it is the tattoo. Upon discovering this Guildenstern tears the tattoo off Sydney's back to obtain the power of Darkness, though he only bears it for a short time. In the end, both Rosencrantz and Guildenstern are dead, and the powers pass to Ashley Riot, a Riskbreaker with no love for the Dark. He becomes a vagrant, and never uses its power for evil.
The Blood-Sin is in the background when the player summons. It is signed by Gabranth in his first major appearance, and an identical Blood-Sin mark to the infamous tattoo from Vagrant Story appears on the skull of the shamaness during Zalera's spell "Death Wail".