Tanaburs
A necromancer who lived in Britain some time between the time of Joseph of Arimathea and Uther Pendragon. He was hailed as the greatest magician ever to reside on the island, save Merlin.
To keep a knight from reaching his unfaithful wife, who lived at Corbenic (Carbonek), Tanabos enchanted the Grail Castle in such a way that it could only be found by accident. The enchantment lasted until the time of Charlemagne, who razed Corbenic.
Sources
Post-Vulgate Queste del Saint Graal | 1230-1240
Prose Tristan | 1230-1240