[1][2][3][4] Rhydderch appears in Adomnán's Vita Sancti Columbae, written around 700, where he sends a secret message to the saint asking him to prophesy the method of his death.
The description of his death is assumed to be accurate, as Adomnán was writing at a time when Rhydderch's life was probably still relatively well known, and he would be unlikely to attribute a false prophecy to St Columba.
In the 9th century Historia Brittonum, Rhydderch is one of four Brythonic kings (along with Urien, Gwallog and Morcant) named as fighting against Hussa of Bernicia.
One of the saint's miracles was to save Rhydderch's adulterous Queen Languoreth from the king's wrath, by rediscovering her lost ring and thereby proving her innocence.
The list also attempts to justify Rhydderch's epithet 'Hael' in describing the sword: "if a well-born man drew it himself, it burst into flame from its hilt to its tip.