text: This man’s spring at Barenton was a popular pilgrimage site.
full_question: This man’s spring at Barenton was a popular pilgrimage site. French sources usually referred to this man’s mother as Optima. If his namesake tree, also known as the Priory Oak, was cut down, then the town of Carmarthen would be destroyed. He pointed out that a pool was why Vortigern’s tower kept falling down. He convinced Aurelius Ambrosius to venture to Ireland to retrieve the stones to build Stonehenge. Igraine slept with Uther Pendragon through the machinations of this man, who then organized the sword-in-the-stone contest. For 10 points, name this magician in Arthurian legends.
first_sentence: This man’s spring at Barenton was a popular pilgrimage site.