The Cottonian Chronicle for the year 954 notes:

Bellum iuxta nant conuy inter filios idwal et filios howel. Edywin filius howel moritur. Anaraud filius Guriat occiditur (Gough-Cooper, c278.1-3)

Translation: A battle near Nant Conwy between the sons of Idwal and the sons of Hywel. Edwin son of Hywel dies. Anarawd son of Gwriad is killed.

The Breviate Chronicle names the battle `Gurguist? (Gough-Cooper, b976.1). The Peniarth 20 verion of Brut y Tywysogyon adds the additional information that the battle was `at the place which is called Gwrgystu: the battle of Conwy Hirfawr? (Jones, 1952, 7), and the Red Book of Hergest version of Brut y Tywysogyon has it as `the battle of the Conway at Llan-rwst? (Jones, 1955, 13).

The sons of Idwal were Iago and Ieuaf, rulers of Gwynedd who had previously fought at Nant Carno (951) and Dyfed (953).

RCAHMW (Battlefields Inventory), Dec 2016

Bibliography
Gough-Cooper, Henry (ed.) The Breviate Chronicle: Annales Cambriae, The B Text from London, National Archives, MS E164/1, pp. 2?26, online edition.
Gough-Cooper, Henry (ed.) The Cottonian Chronicle: Annales Cambriae, The C Text from London, British Library, Cotton MS Domitian A. i, ff. 138r?155r, online edition.
Jones, Thomas (trans.), Brut y Tywysogyon or The Chronicle of the Princes Peniarth MS. 20 Version (Cardiff, University of Wales Press, 1952).
Jones, Thomas (ed. and trans), Brut y Tywysogyon or the Chronicle of the Princes: Red Book of Hergest Version (Cardiff, University of Wales Press, 1955).