This rune was borne by Bob during the Dunan Unification War and was how the werewolf was able to maintain a human body. During battle, Bob could weaken the rune, allowing himself to transform into a full werewolf. That said, since werewolves were extremely rare, whether that was the case or that the rune cursed him to become a werefolf can not be said with absolute certainty.
Regardless, the transformation would turn Bob into a savage, powerful beast. However, when the rune reasserted itself, he would be left weakened.