The chapter is an edition and discussion of the medieval Welsh chronicle O Oes Gwrtheyrn. Although a relatively short text which has seen little discussion in the past, O Oes Gwrtheyrn is notable in several important ways. Firstly, it is the only monastic chronicle to survive from medieval Gwynedd, the most politically important kingdom of medieval Wales. The edition establishes that the text most likely derives from the Cistercian monastery of Aberconwy, and was first compiled in the second decade of the thirteenth century. Secondly, the likelihood that this chronicle was written originally in Welsh mean it was an innovative new departure in vernacular historical writing. This gives the short text a wider significance not only in the development of medieval chronicles in Wales, but also in terms of the development of European vernacular historical writing and the relationship of international monastic orders such as the Cistercians to this process.
The third notable aspect of the chronicle is its unique recording of some events. Although much of the material contained in the chronicle is drawn from other sources, its fuller account of some years in the early thirteenth century offers a unique perspective on known events and it records some otherwise unattested events. The most notable of these is the death of the Norwegian Erlendr Pikr at Llanfaes in 1209 which can be considered the last viking raid on Wales, and which brings Wales briefly into the history of the Norwegian civil wars.