He revealed that two warring underground dragons were foiling Vortigern's efforts to build a tower, and helped another king enter Tintagel disguised as his rival (*)  
This figure addresses apple trees and a pig in a pair of poems from the Black Book of Carmarthen. He and Joseph of Arimathea are the title characters of the only surviving poems by Robert de Boron, while other authors conflate him with Aurelius Ambrosius. He revealed that two warring underground dragons were foiling Vortigern's efforts to build a tower, and helped another king enter Tintagel disguised as his rival (*)  Gorlois in order to sleep with Igraine. He dies after falling in love with, revealing his secrets to, and being trapped in a rock or tree by a woman identified as Viviane, Nimue, or the Lady of the Lake. He included an empty seat called the Siege Perilous in his design for the Round Table. For 10 points, identify this wizard who advised King Arthur.
This figure addresses apple trees and a pig in a pair of poems from the Black Book of Carmarthen.
Merlin  [or  Myrddin ; accept  Merlin us Ambrosius or  Myrddin  Emrys or  Myrddin  Wyllt]