Helig ap Glanawg (standard modern Welsh orthography: Helig ap Glannog) was a 6th century prince who lived in North Wales.
It is said that the river Conwy once reached the sea by the Great Orme, Llandudno, and to the west lay the great cantref of Gwaelod which stretched all the way to Puffin Island, off Anglesey.
Helig ap Glanawg lived here, but in the 6th century all his land was inundated by the sea, this forming the Lavan Sands which lie between the Great Orme's Head and the Menai Straits off the north coast of Gwynedd. This has given rise to the legend of the drowned kingdom. It is said that the remains of Llys Helig, his palace, can be seen at exceptionally low tides, this being near the Conwy channel, about a mile or so off the coast at Penmaenmawr.
Since then, the story has been embroidered into various myths, such as the flood being the result of revenge after Helig ap Glanawg's daughter was unfaithful in love.
Llys Helig is mentioned in a number of old documents, some of were published after expeditions to find Llys Helig.
After the disaster both Helig and his numerous sons embraced a religious life. These sons, according to various sources, were -
Celynin, who has a church dedicated to him at Llangelynin in the Conwy valley, and there is another at Llangelynin near Llwyngwril, a few miles south of Llanaber.
Rhychwyn, the saint associated with Llanrhychwyn church.
Bodfan, to whom the church at Aber, in Gwynedd, is dedicated.
Brothen, who founded the Church at Llanfrothen
Peris, who founded the churches at Llanberis and Nant Peris.
Boda and Gwynin, who founded the church at Dwygyfylchi, near Penmaenmaw
(Wikipedia)