What is "tyfrydog"

Tyfrydog (sometimes given as Tyvrydog) was a Christian from north-west Wales in the fifth or sixth century, who was later venerated as a saint. He is said to have established a church in Anglesey, and although no part of the original structure remains, the current church is still dedicated to him. A nearby standing stone is said to be the remains of a man who he punished for stealing a bible from the church.