During the reign of King Jinheung he was in charge of certain ceremonies and prayers. He was noted for his knowledge of astrology and prowess in swordsmanship.
He received a revelation that when the stars of the Big Dipper becomes eight, one who will be able to go against Lady Mishil will exist. He thus protects the other twin baby Deokman by helping Sohwa escape from ChilSuk.
King Jinji entrusted his son Hyeomjeong to Munno, and he renamed him Bidam. He raised the child intending for him to someday become the king of Shilla and unite the Three Kingdoms (Baekje, Goguryeo and Shilla). He also planned to marry Deokman and Bidam, having planned on raising them both together after meeting back up with Sohwa.
However, Sohwa, not wishing for Deokman to be dragged into such a life flees with her, and hides away.
Munno educates Bidam as a student. He taught Bidam his style of swordsmanship, but kept certain techniques away from him. He was shocked when he saw that Bidam killed dozens of people by trying to protect the books of the Maps of the Three Kingdoms, not understanding that Bidam simply wanted to make him happy. From that moment he feared Bidam, believing him to be cold-blooded and ruthless, and thus, unfit to be the one to unify the 3 Kingdoms as King.
He then began to believe that Kim Yushin was the real chosen one.
Munno was shot by a poison needle while battling against Bidam and eventually died. Bidam carried him on his back, but it was too late. Munno's final words to Bidam were for him to become a Hwarang and that he officially considered Bidam as his student.
Munno was killed by an assassin sent by Yeomjeong who found out that Munno was trying to give the Maps of Three Kingdoms that they made to Kim Yushin, whom he did not know.
Bidam never reveals Munno's death so that he would remain as the legendary protector of Shilla.