Queen's Debt

Future AU, what happens when SwanQueen has to save the day because of a blood debt of Regina's from childhood? And, how does it relate to a prophecy about bringing Avalon back to its former glory? And, just who is Rhiannon, Queen of Tir Nan Og? These are the questions that Emma and Regina have to figure out if they want to go back to their quiet life in Storybrooke.