From Final Fantasy Wiki

The Rood of Iocus, an inversion of the Blood-Sin.

The Blood-Sin is a tattoo of great power in Vagrant Story. Whoever bares it upon their back has the power to control the Dark and the city of Leá Monde. It was firth born 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". This symbolizes their aversion of the Darkness the cult had worshiped. The priesthood had since labeled the Blood-Sin symbol as the "Rood Inverse".

Somehow, the lineage of Müllenkamp survived, and the Blood-Sin has since passed through the family of Duke Bardorba. Sydney Losstarot, the Duke's son, holds the Blood-Sin for much of Vagrant Story, and his main goal is to pass it off to another who will not abuse its Dark powers. Both Jan Rosencrantz and Romeo Guildenstern both desire the power of the Dark, and so search for "the Key", not knowing it was this tattoo all along. Guildenstern goes as far as to tear the tattoo off Sydney's back so as 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 its powers instead pass to Ashley Riot, a Riskbreaker with no love for the Dark. He becomes a vagrant, and never uses its power for evil.