The manga suggests a bit of a darker history. Luka tells Yuki he was considered a slave because he belonged to the Crosszeria clan (also known as the "Clan of the Traitor"), and that all the other Duras were intent on punishing the whole family and all its descendants. The demon king owned Luka, and as a symbol of this, he formed a permanent covenant by putting his own blood into Luka's Brand Zess. The demon king's blood made the mark on Luka's arm glow red, and it caused Luka a great deal of pain during the process. Years later, Luka is still haunted by the memory and feels physically ill when recalling it. Because it was a covenant, if Luka should ever betray the demon king, he would die. Luka explains to the current incarnation of Yuki that since he made a new contract with Yuki, the previous contract with the demon king was nullified, allowing Luka to live. However, Luka hints that the prior contract with the demon king may not be completely nullified.