J. Beverley Smith writes:
“Intimations of treachery, of breach of faith, are so often conveyed darkly, and no chronicle, nor any other source, provides the unequivocal teestomny which might enable us to unravel the threads in the various accounts of the tragic happening in the vicinity of Builth. It was alleged at the time, or shortly afterwards, in the most explicit statement we have, that the prince’s decision to venture into the area was influenced by one of the sons of his old adversary, Roger Mortimer. The Hagnaby chroinicler, an important source for the events of the day on which Llywelyn died, was quite definite: Roger Mortimer, he says, but, more correctly, his brother Edmund Mortimer, drew the prince there by beseeching him to come to the neighbourhood of Builth to take his homage and that of his men. Along with other lords he hatched a plot to corner Llywelyn and kill him” (Llywelyn ap Gruffydd, 1998:551).
The chronicle of Hagnaby Abbey is a historical document that begins in 1173 with the foundation of the Abbey in Lincolnshire. It is now ruined. It was a house Premonstratensian canons, “founded in 1175-1176 as a dependency of Welbeck Abbey. It gained independence and abbey status in 1250, and was supressed in 1536.” http://www.pastscape.org/default.aspx
Whatever really happened, the entry from the Chronicle of the Princes (Ystrad Flleur) says it all:
And then Llywelyn ap Gruffudd left Dafydd, his brother, guarding Gwynedd; and he himself and his host went to gain possession of Powys and Buellt. And he gained possession as far as Llanganten. And thereupon he sent his men and his steward to receive the homage of the men of Brycheiniog, and the prince was left with but a few men with him. And then Edmund Mortimer and Gruffudd ap Gwenwynwyn, and with them the king’s host, came upon them without warning; and then Llywelyn and his foremost men were slain on the day of Damasus the Pope, a fortnight to the day from Christmas day; and that was a Friday.
—-Brut y Tywysogyon, Peniarth manuscript 20 (The Chronicle of the Princes)
His head was carried to King Edward I, who ordered that it be displayed on a pike, in London. Apparently, it stayed on display for over 20 years. The rest of his body is purportedly buried at Abbey Cwmhir, northeast of Rhayader in Powys.
Llywelyn’s brother, Dafydd, was eventually captured and hanged, drawn, and quartered, the first man of significance to experience that particular death. His death was practice for what Edward did to William Wallace, two dozen years later. Gwenlllian, Llywelyn’s daughter and only child, was kidnapped from Aber and sent to a convent in England, where she remained a prisoner her entire life.