Gvenhvyuar ("White phantom" or Gwenhwy-mawr "Gwenhwy the great" in contrast to her sister Gwenhv(/w)y-(v)ach mentioned in Culhwch ac Olwen*; Geoffrey of Monmoth [see Other Works] gives the possibly older Guenhuuera/Guanhumara*). The Bruts and two triads (see Other Works) have her as the daughter of (G)ogvran Gawr ("(G)ogvran the giant")*.
It seems that the most significant role Gvenhvyuar plays in the Arthurian story is through her abduction and the possibly resulting battle, which ended Arthur's life. In Caradog's Vita Gildae Gildas arrives at King Melvas' city of Glastonbury to find it under siege from Arthur with the armies of Cornwall and Devon. Arthur is fighting for the return of Guennuvar, whom Melvas has abducted. Gildas negotiates the return of Arthur's wife. This story almost certainly antedates Geoffrey of Monmouth* and it is interesting in giving Arthur a South-Western command similar to many of the older sources. Geoffrey has Guanhumara abducted by Arthur's nephew Modred (Medrawt in the Bruts)*.
From the triad "the three Unrestrained Ravagings of the Island of Britain"* we learn that Medrawd came to Arthur's court at Celliwig in Cornwall, drank and ate everything, then he dragged Gwenhvyuar from her chair and hit her - though we don't actually know whether this refers to the abduction implicitly. Either way Arthur went to Medrawd's court in turn and took everything there.
With regard Anir, a few points are worth noting about the abduction. The first is that both Modred, the son of Arthur's sister Anna (see Eigr), and Amr appear to have gained Arthur's displeasure and been killed by him in battle, possibly suggesting they are one and the same. The shift from wife-abducting son to wife-abducting nephew might also be present in the story of Tristan and Isolde (see Kynuavr). The second is that it has been speculated that Amr was regarded by Arthur as a lower-class citizen because he was illegitimate. More likely, one might have thought, that if true it would be because he was the result of the abduction of Gvenhvyuar, though plainly this can't be true if our first speculation is. The third is the pairing of good-bad siblings within the Arthurian family: Medrawt-Gwalchmei (see Eigr) and Amr-Llacheu.
Anyway, to continue: from the triad "the Three Harmful Blows of the Island of Britain"* we learn that Gwenhwyfar was hit by her sister Gwenhwyfach, and that this caused the battle of Camlan(n) This quarrel-inspired battle was one of "the Three Futile Battles of the Island of Britain"* - the Annales Cambriae (See Other Works) list its date as 537 CE, and note that both Arthur and Medraut died there. Sites for the battle have been speculated as Camboglanna/Birdoswald on Hadrian's Wall or the River Camel in Cornwall*, though there are a vast number of suggested alternatives of varying sanity. Given the Modred-Amr similarity, it is tempting to add the Gamber site of Wonder 13 to them (it has more going for it than most), though to do so would be to elicit, quite rightly, the collected groans of Arthurian scholars worldwide.
Wherever it was, the triad "the Three Unfortunate Counsels of the Island of Britain"* implies that Arthur failed to win the battle because he divided his men into three units and possibly gave some to Medrawd, perhaps though the plotting of Idawc Cord Prydein ("Idawc, Agitator of Britain")*, who in the Welsh tale "the Dream of Rhonabwy" (English translation) claims to have altered messages between the two. The fact noted in the triad "the Three Faithless War Bands of the Island of Britain"* that the war band of one friend slid off the night before, cannot have helped the situation.
Either way, Post-Geoffrey the Bruts lay the battle not at the feet of a sibling spat between Gwenhwyfar and Gwenhwyfach, but at the feet of treacherous Medrawd with foreign allies, essentially saying he claimed the kingdom while Arthur was fighting the Roman Emperor abroad. This is despite early Bardic sources have him as a generally good person. Indeed, he is included in the list "the 24 Knights of Arthur's Court"* as one of the three Royal Knights whom no one could refuse on account of their beauty and wisdom.
Of course, all this confusion is not entirely irreconcilable - perhaps Gwenhwyfar fell, Stockholm-like, for pretty-boy Medrawd, a fact only revealed upon her return by her sister. She is, after all, listed in the triad the "Three Faithless Wives of the Island of Britain". Still, it all adds to the general confusion of the Arthurian picture.