A knight who lived almost a century before Arthur’s time.
His father, King Assen, strongly disapproved of Anasteu’s love for a lowborn woman. Fleeing his father’s wrath, Anasteu took his lady into a forest, and carved a home for them out of solid rock, equipping it with various luxuries.
Later, this cavern served as Merlin’s prison after he was shut up by the Lady of the Lake.
Source
Vulgate Merlin | 1220-1235