page 37 of 960 of Sir Thomas Malory: Le Morte Darthur: The Definitive Original Text Edition

037:

The questing beast was born of an incestuous union between a brother and sister (whom the devil arranged) and this juxtaposes with Arthur’s incest. A cursed knight is forced to chase this beast, analog with Sisyphus. All this points to cracks in the foundation of Arthur’s realm.

Ulphuns insults Arthur’s mother, Queen Igrayne (+ Merlin) for not revealing Arthur’s parentage thus leading to the incest.