Ahh, it's so unknown here it doesn't even have an article in Polish Wikipedia. Memory holed.

Manipulated by the pope, a young king broke a peace treaty with the Ottomans against the opposition of the nobles (who thought it was dishonorable), then was killed in the resulting total disaster aged just 20 after being betrayed by the Venetians in this idiotic adventure. There was a legend about how he survived but was too ashamed so lived in hiding.