A king who ruled the Perilous Forest a century or so before Arthur’s time.
His son, Anasteu, loved a peasant woman, which sparked Assen’s ire. Anasteu fled his father’s wrath and carved a home for himself and his lady in a rock. This cavern was later the final resting place of Merlin, when he was imprisoned there by the Ninianne.
Source
Post-Vulgate Suite du Merlin | 1230-1240