Gwenhwyfach (Middle Welsh: Gwenhwyvach, Middle Welsh: Gwenhwywach, or Middle Welsh: Gwenhwyach; 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 relatively 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.
Rachel Bromwich notes, citing the spelling found in Culhwch and Olwen and Triad 84, that Gwenhwyach may in fact be the original spelling of the name. Melville Richards and Bromwich previously suggested that the alternate spelling of her name in medieval Welsh sources, Gwenhwywach, could have been 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". It is possible that Gwenhwyfach was once thought of as a darker aspect of Gwenhwyfar.
The Lancelot-Grail cycle introduced a possibly related character known as "the False Guinevere" or "Guinevere the False", also known as the Lady of Camelide (Dame de Camelide), the real Guinevere's evil half-sister who bewitches Arthur.
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. In Bernard Cornwell's Enemy of God (1996), she is a delusional and dim-witted Guinevere's fat sister who aids Arthur in his supposed "rescue" of Guinevere from Lancelot's castle, and later goes completely insane while living alone waiting for her own prince to arrive.
- Bromwich, Rachel, Trioedd Ynys Prydein, 3rd Ed. University of Wales Press, 2006, p. 376.
- Richards, Melville, "Arthurian Onomastics", in: Transactions of the Honourable Society of Cymmrodorion, vol. 2, 1969, p. 257.
- Collins, Morris. "The Arthurian Court List in Culhwch and Olwen". The Camelot Project at the University of Rochester. Retrieved 3 January 2013.
- Ziegler, Michelle (1999). "Brigantia, Cartimandua and Gwenhwyfar". The Heroic Age (1). ISSN 1526-1867. Retrieved 8 December 2012.
According to Patrick Sims-Williams, in Welsh the "termination of -ach evokes unpleasantness" (Sims-Williams 1991:42). Therefore, Gwenhwyfar's sister Gwenhwyfach, found in the Welsh triads (Bromwich 1978) and Culhwch and Olwen (Ford 1977:131), may represent an unpleasant or evil form of Gwenhwyfar herself.