For 10 points, name this figure who tells Uther Pendragon to found the Round Table and proposes the test of the sword in the stone; a bard and magician of Arthurian legend.
This man lived in a house with seventy doors and seventy windows built by his sister, Ganieda. In his other invisible house of glass, he guards the Thirteen Treasures in penance for being seduced by Nimue. He prevented himself from being sacrificed by King Vortigern by leading the king to two dragons, whose fight represented the clash of Britons and Saxons. He was fathered at Carmarthen by an incubus; later, he enables Ygerna to become pregnant. For 10 points, name this figure who tells Uther Pendragon to found the Round Table and proposes the test of the sword in the stone; a bard and magician of Arthurian legend.
This man lived in a house with seventy doors and seventy windows built by his sister, Ganieda.
{Merlin}