It was Danish King Valdemar II, who had the flag fall from the sky to him as a sign. Some historians say it was a cross-battle dream, like Constantine, or like the 1217 Seige of Alcacer do Sal.

I guess hallucinating crosses during or after battles was just a common thing for a while. Everyone who lost while hallucinating a cross probably died so there's probably some confirmation bias there too. It's like how praying for your team to win a superbowl only works if you end up winning, if you lose then I guess the other team prayed harder.