Rivallo (Welsh: Rhiwallon) was a legendary king of the Britons as accounted by Geoffrey of Monmouth.
[1] He was the son of King Cunedagius and was noted as a young king who reigned with moderation.
Geoffrey describes him as a "peaceful and fortunate youth, who ruled the kingdom well".
[2] His reign was troubled by natural disasters: a rain of blood that lasted three days, a devastating plague, and a great swarm of flies.
[2] He was succeeded by his son Gurgustius.