King Benlli was a British king who ruled part of what is now Wales in the early 5th Century AD. He is notorious for opposing Saint Germanus and was probably a heretical follower of Arianism. The story of his admonishment by the saint and eventual demise by "fire from heaven" is recorded in Historia Brittonum, chapters 32-35. The hill fort at Foel Fenlli is traditionally considered to have been his castle.