Wikipedia
Gwenhwyfach (; sometimes anglicized to Guinevak) was a sister of Gwenhwyfar (Guinevere) in medieval Welsh Arthurian legend. The tradition surrounding her is preserved in fragmentary form in two Welsh Triads and the Mabinogi tale of Culhwch and Olwen.
This obscure figure is first mentioned in Culhwch, where her name (spelled Gwenhwyach) is among those 200 men, women, dogs, and horses invoked by the hero Culhwch to punctuate his request that King Arthur help him find his love Olwen.
Both of the Triads that mention Gwenhwyfach refer to 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 that gave her sister that caused the Strife of Camlann. Identifying Camlann as one of Britain's "Three Futile Battles", Triad 84 mentions it was started because of a dispute between the sisters. Some have suggested that "Gwenhwyfach" in Triad 53 is a mistake for " Medrawd" (Mordred), since Triad 54 describes Medrawd raiding Arthur's court and throwing Gwenhwyfar to the ground and beating her; this interpretation does not explain Triad 84, however.
As noted by Welsh scholars Melville Richards and Rachel Bromwich, her name can be understood as Gwenhwy-fach, or "Gwenhwy the Lesser", a back-formation based on a false etymology of her sister's name as Gwenhwy-fawr, meaning "Gwenhwy the Great".
The Lancelot-Grail cycle includes a possibly related character known as "the False Guinevere" or "Guinevere the False", a half-sister of the real Guinevere whose claim to be the real Guinevere is for a time accepted by Arthur. It is possible that Gwenhwyfach was once thought of as a darker aspect of Gwenhwyfar. Some modern writers associate Gwenhwyfach with Mordred, presumably due to her association with Camlann; she appears as the traitor's wife in Thomas Love Peacock's novel The Misfortunes of Elphin (1829), for example.