The Venedotian Code of the
Welsh laws compiled by Iorwerth ap Madog in the early 13th century contains a list of the privileges of the men of
Arfon. Among the privileges is the right to march in the van of Gwynedd's army, and this is stated to originate from their spirited actions in a war between Rhun of Gwynedd and the Cymric Men of the North () from the kingdoms of
Alt Clut and
Gododdin or
Manaw Gododdin.
Taliesin's
Marwnad Rhun () also tells of the war and Rhun's death in it. In his comprehensive discussion of the works by and attributed to
Taliesin,
John Morris-Jones notes that the particulars of the
marwnad are everywhere consistent with the historical record and nowhere inconsistent, and likely a product of the 6th century, a view shared by notable skeptics such as
Thomas Stephens. The Venedotian Code says that the northern prince Elidyr Mwynfawr ap Gorwst Priodawr () had been slain at Aber Mewydus (now called "Cadnant", or "Battle Brook") in Arfon, not far from Rhun's
llys () at Llanbeblig. Elidyr's powerful relatives in the North invaded Gwynedd in retaliation, burning Arfon in the process. The Northern host was led by
Clydno Eiddin; Nudd the Generous, son of Senyllt;
Mordaf the Generous, son of Serfan; and
Rhydderch Hael, son of
Tudwal Tudelyd. These are all notable men of the era who are listed in the royal genealogies of the
Bonedd Gwŷr y Gogledd, as is Elidyr Mwynfawr. According to one of the
Triads of the Horses, Elidyr was also the husband of Rhun's sister Eurgain. in the North. The final outcome is not given in the Venedotian Code, but Rhun and his army remained in the North for a considerable length of time. The outcome according to Taliesin's
Marwnad Rhun is the death of Rhun ap Maelgwn in battle. He was succeeded by his son
Beli ap Rhun as King of Gwynedd. The reason why Elidyr was in Gwynedd and the circumstance of his death are not known, though it is certain from their actions that his northern relatives blamed someone in Gwynedd. There are later stories that add speculations, for example by asserting that Elidyr was contesting Rhun's succession to Maelgwn Gwynedd's throne,{{Citation |last1=Baring-Gould |first1=S. |last2=Fisher |first2=John |year=1908 |contribution=S. Elidyr Mwynfawr == Caerhun ==