Mabuz was the son of the water fairy (the queen of Maidenland) who had raised Lancelot. Within Mabuz's domain was a beautiful forest called, appropriately, Beautiful Wood or Beforet, but he could not enjoy it because his neighbor, the undefeatable Iweret, had annexed it. For this reason, his mother (the fairy) charged Lancelot to defeat Iweret in combat.
Mabuz so loathed courageous knights that he had his castle enchanted in such a way that any knight who entered uninvited would turn into a complete coward. He imprisoned these bewitched warriors and killed them on occasion, whenever he was in a bad temper. Lancelot happened upon the castle during his adventures and succumbed to the spell. Mabuz beat him and threw him in prison with his other knights.
When Iweret became intolerable, however, and began burning Mabuz's lands, Mabuz went to his prison to look for the most cowardly knight there, knowing that, with the enchantment lifted, that knight would be the bravest. He picked out Lancelot, who agreed to fight Iweret provided that Mabuz refrain from killing any of his prisoners for a year. Mabuz agreed and freed Lancelot, who ended Mabuz's troubles by slaying Iweret.