Core Q&A Expanded Answer
During the beach parlay, Breanna restores Mjölnir to Thor after having removed it through time and space at the end of Book Three. The moment is witnessed by Celtic tribal leaders, their warriors, the Warriors of Destiny, and the Great Wyrms of Pretannia and Erin. The latter become The Mórrigan's Raven Wings, who rule the Northern Bloc's skies.