Board Thread:Questions and Answers/@comment-27724978-20190529011049/@comment-25546778-20190602230742

I think it could mean that "the Prince/Princess who was promised" refers to Ice and Fire. Daenerys wasn't fire, as Jon Snow wasn't ice. Jon Snow/Aegon Targaryen was a prince of fire and killed Daenerys in order to destroy the coming darkness that Daenerys was to unleash upon the world.

Arya Stark, since the North  was still independent, that makes Arya a princess. The princess who was promised was ice. She killed the Night King, just like the "last hero" fought the White walkers during the Long Night.

The prophecy refered to TWO heroes, not just one.