Gwenhwyfach, also spelled Gwenhwyvach or Gwenhwyach, is a sister of Gwenhwyfar (Guinevere) in early Welsh Arthurian legend. Most of the tradition surrounding her has been lost; she is mentioned only in two of the Welsh Triads and in the Mabinogion tale Culhwch and Olwen. As Richard Melville and Rachel Bromwich note, the name Gwenhwyvach may be interpreted as "Gwenhwy-fach", or "Gwenhwy the Lesser", a back-formation derived from an incorrect interpretation of her sister's name as "Gwenhwy-fawr", "Gwenhwy the Great"[1][2].
Gwenhwyfach first appears in Culhwch, where her name is spelled Gwenhwyach and she is just one of the 200 men, women, dogs, and horses that the hero Culhwch invokes to punctuate his request that King Arthur help him find his love Olwen. Both Triads that mention Gwenhwyfach speak of the enmity between her and her sister that led to the Battle of Camlann. Triad 53 lists as one of the "Three Harmful Blows of the Island of Britain" the slap Gwenhwyvach gave to her sister that caused the Strife of Camlann. In calling Camlann one of Britain's "Three Futile Battles", Triad 84 mentions it was started because of a dispute between the sisters.
Triad 54 describes Medrawd (Mordred) raiding Arthur's court and throwing Gwenhwyfar to the ground and beating her, leading some to suggest Gwenhwyfach was a mistake for Medrawd in Triad 53. This interpretation does not explain Triad 84, however.
The Lancelot-Grail cycle includes a character known as the "False Guinevere", the half sister of the real Guinevere whose claim to be the real Guinevere is for a time accepted by Arthur. Gwenhwyfach's association with Camlann has made some modern writers associate her with Mordred as well. She appears as the traitor's wife in Thomas Love Peacock's The Misfortunes of Elphin (about Elffin ap Gwyddno).