A supreme wizard who lived on the Misty Lake in Ulrich von Zatzikhoven’s Lanzelet.
He was a sworn enemy of Arthur, since Erec had killed his father, Gawain had killed his brother, and Arthur had driven him out of Britain. Even so, when Arthur needed help rescuing Guinevere from the clutches of King Valerins of the Tangled Wood, he called on Malduc. Malduc agreed to lift the enchantment prohibiting their entry into Valerins’ castle provided that Arthur turn over Erec and Gawain to him. Arthur reluctantly agreed – knowing that Erec and Gawain would give themselves gladly for the Queen – and Malduc caused the knot of brambles and branches that surrounded Valerins’ fortress to unwind.
Once he had his hands on Erec and Gawain, he locked them in cages and delighted in torturing them daily. Finally, when word of their torment had sickened Arthur’s court long enough, Lancelot led an expedition to rescue the two knights. With the help of the giant Esealt the Tall, Lancelot and his men got into Malduc’s sanctuary on the Misty Lake, killed him and his household, and freed their comrades.
He had a single unnamed daughter who was allowed to live.
Source
Lanzelet | Ulrich von Zatzikhoven, c. 1200