Nant Gwrtheyrn ('the valley of Vortigern') is outside of the small town of Llithfaene on the Lleyn peninsula in North Wales. Nant Gwrtheyrn is the supposed grave of Vortigern.
It is not clear why this this valley was named Nant Gwrtheyrn - it is only one in Wales named after Vortigern. Vortigern's legends do take him to north Wales (especially Snowdonia), but never to the Lleyn peninsula itself.
Around 1700 AD a stone grave covered with a turf mound was found here, which was called Vortigern's Grave (Bedd Gwrtheyrn) by the local population. George Borrow, writing in 1862 (Wild Wales), described it as "It was in a wind-beaten valley of Snowdon, near the sea, that his dead body decked in green armour had a mound of earth and stones raised over it".
Arthurian Sites in Wales