The Once and Future King
When Brynden Rivers ("Bloodraven") promises to do everything in his power to ensure Jon Snow is able to thrive before the Night King Comes, he sets events in motion which place the pair in the midst of love, death, war, and betrayal. With his peculiar life-cycle, he will ensure the once promised prince can be the future king.
(i.e. Brynden is Merlin and Jon is Arthur)