What finally convinces Leodogran of Arthur's nobility in Idylls of the King?

Leodogran has a dream of a king standing triumphantly in the heavens above the roiling masses and knows it to be a sign of Arthur's true kingship. When he wakes up, he sends word to Arthur that Guinevere's is his to marry. Arthur sends his most beloved knight, Lancelot, to retrieve Guinevere and they are married thereafter.