Overview
Gaheris (Old French: Gaheriet, Gaheriés, Guerrehes, etc.) is a Knight of the Round Table in the chivalric romance tradition of Arthurian legend. A nephew of King Arthur, Gaheris is the third son of Arthur's sister or half-sister Morgause and her husband Lot, King of Orkney and Lothian. He is the younger brother of Gawain and Agravain, the older brother of Gareth, and half-brother of Mordred. His figure may have been originally derived from that of a brother of Gawain in the early Welsh tradition, and then later split into a separate character of another brother, today best known as Gareth. German poetry also described him as Gawain's cousin instead of brother.
Malory's Le Morte d'Arthur depicts Gaheris as little more than a supporting character to Gawain, with an odd exception of his murder of their mother. However, his role is greater in French prose cycles that were Malory's sources, including as an object of murderous sibling rivalry by his older brother Agravain in the Vulgate Cycle. Inevitably, both there and in Malory, Gaheris is killed alongside his other brother Gareth during Lancelot's rescue of Guinevere, the event that will lead to the fall of Arthur.
Origin
Gaheris and his brother Gareth likely originated from the same character, due to the similarity of their names in Old French (where the many forms of their names include such as the almost entirely exact Guerrehes and Guerrehet). Furthermore, the adventures ascribed to the brothers are often interchangeable and indistinguishable. Only one brother is ever named for Gwalchmai ap Gwyar, the figure from Welsh mythology traditionally identified with Gawain. This character, a prince named Gwalchafed [Gwalhafed] or Gwalhauet [Gwalhavet] (Old Welsh for "Hawk of Summer") ap Gwayr or mab Gwyar, mentioned in Culhwch and Olwen, is a likely common source for both Gaheris and Gareth, if Gawain was indeed derived from Gwalchmai. A later French-influenced Welsh romance Seint Greal does in fact call Gwalchmei's brother Gaharyet.
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