This poem, Dialogue between Myrddin and his sister Gwenddydd, is from the LCH. It is taking place after the battle of Arderydd of 575. It then conflates Rhydderch Hen, who may have participated in that battle, with Rhydderch Hael of Gwynedd. The dialogue refers to his victory at the ford of the river Towy. This river flows through Caerfyrddin before entering Carmarthen Bay.
BGG 8 gives no name as successor to Rhydderch, see Rhydderch Hael and Rhydderch Hen, but the poem tells us it was Morgan Mawr ap Sadyrnin. The dialogue states Gwenddoleu, who was of gen. 1, see the table below, would have been the natural successor as supreme British monarch but for his death at Ardderyd. Sadyrnin was St. Sadwrn and there is a Llansadwrn both in Carmarthenshire and in Anglesey. A stone bearing his name in Latin, Saturninus, was found at the parish church in Anglesey. Nash-Williams states Sadwrn Farchog was the brother of St. Illtyd. Their mother was Rieingulid, daughter of Amlawdd Wledig, according to the V. Illtuti. This allows us to conclude Morgan also belonged to gen. 1. The poem goes on to say Morgan was suceeded by Urien. As he too belonged to gen. 1, it may have been that Urien replaced Morgan as leader of the British forces. This may have been the reason behind Urien’s assassination. Bartrum had difficulty reconciling this sequence of three kings and suggested the sequence was in reverse order. However, this was not the case and his mistake was to assume that Rhydderch Hael and Rhydderch Hen were the same individual.
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At this point the poem goes back a generation and begins to give the pedigree of Gwynedd, starting with Maelgwn, largely in conformity with that given in HG 1, see The king-list of Gwynedd. The Venedotian sequence ends with Anarawd ap Rhodri Mawr.