Triad 68—"Three Kings who Sprang from Villeins"—lists Hyfaidd among their number, meaning that his father Bleddri or Bledrig was held to have been a serf rather than a member of Dyfed's old royal family claiming descent from Aed Brosc. An alternate theory relates him to the royal family of Ceredigion. His mother was supposed to be Tangwystl, a daughter of the earlier King Owain.
Charles-Edwards argues that Hyfaidd was responsible for consolidating the lands that would later become Deheubarth, annexing Ystrad Tywi and possibly Ceredigion to Dyfed before his death. He was said to have oppressed the clerics of Meneva (modern St. David's) and exiled Bishop Nobis, earning him the enmity of Nobis's kinsman, the historian Asser.
Although later Welsh histories made Hywel Dda's inheritance of Dyfed a peaceful affair brought about by his marriage to Hyfaidd's granddaughter Elen and the extinction of Hyfaidd's male line, Asser's more contemporary Life of King Alfred reports that Dyfed or Brycheiniog both fell under such sustained attack from Hywel's uncle Anarawd and father Cadell that Kings Hyfaidd and Elise submitted to King Alfred of Wessex's overlordship in exchange for protection.
- Wolcott, Darrell. Ancient Wales Studies: "The Legendary Kingdom of Seisyllwg". Accessed 20 Feb 2013.
- "Three Kings who were [sprung] from Villeins (meibion eillion): Gwriad son of Gwrian in the North, and Cadafel son of Cynfeddw in Gwynedd, and Hyfaidd son of Bleiddig in Deheubarth." The Red Book of Hergest, Triad 59.
- Charles-Edwards, T. Wales and the Britons, 350–1064, p. 495. Oxford University Press, 2012. Accessed 20 Feb 2013.
- Charles-Edwards, p. 489.
- Charles-Edwards, p. 452.
- Thornton, David. Kings, Chronicles and Genealogies: Studies in the Political History of Early Medieval Ireland and Wales, p. 110. Occasional Publications UPR, 2003. Accessed 20 Feb 2013.