Mount Dolereus
An enchanted hilltop, possibly in Scotland. At the top was a pillar, adorned with fifteen crosses, to which only the best knights could tie their horses. Any other knight would be driven insane.
Merlin had constructed the test at the behest of Uther Pendragon in order to find worthy knights for Arthur’s table. Kahedins vowed to travel there in Chrétien’s Perceval, but it is Perceval who tests himself at the pillar and succeeds in the Second Continuation. Merlin’s daughter inhabited the mountain and explained the adventure to Perceval.
Sources
Perceval, or Le Conte del Graal | Chrétien de Troyes, late 12th century
Second Continuation of Chrétien’s Perceval | Attributed to Wauchier of Denain, c. 1200