For obscure purposes, Merlin made a bed to lie in which caused madness. Possibly there is some confusion here with the bed in the Castle Adventurous of Carbonek, sleeping on which insured a person visions of more or less peril.
A marvel encountered by Gawain at the Island of Marvels. Laying on the bed made a man temporarily lose “mind and memory”.
See also
Meliot | The Legend of King Arthur
Perilous Bed | The Legend of King Arthur
Source
Vulgate Lancelot | 1215-1230