The main source of evidence for Pengwern’s existence is contained in two 9th Century Welsh poems dealing with Cynddylan’s death and the destruction of his kingdom at the hands of King Oswiu of Northumberland, thought to have taken place in the year 656. In the past, both Marwnad Cynddylan (the Death Song of Cynddylan) and the Canu Heledd (The Laments of Heledd – in legend, Cynddylan’s sister) have been attributed to Llywarch Hen (Lewis The Old), the last king of the former Welsh state of South Rheged. Llywarch, who is believed from around 534 to 634, became renowned as a poet but it is unlikely he wrote either of the works concerning Cynddylan’s exploits. It is probable that Marwnad Cynddylan and the Canu Heledd were passed by word of mouth for up to 200 years before being written down, which also makes their validity as historical evidence problematical.
However, other evidence, contained in surviving Welsh pedigrees of the period, suggests the actual existence of characters mentioned in Marwnad Cynddylan and the Canu Heledd, while the children of Cyndrwyn Fawr (including Heledd herself) are listed in another contemporary document, the Bonedd yr Arwr. There is also fragmentary evidence to suggest Cynddylan’s involvement in the Battle of Maes Cogwy (Oswestry) in 642, when King Oswald of Northumbria was slain at the hands of King Penda of Mercia. Indeed, it was Cynddylan’s participation in this campaign that supposedly resulted in his own death, fourteen years later, when, after defeating Penda, Oswald’s brother, Oswiu, wrought revenge on the leader of Pengwern and several of his brothers.