Fics like Carmen Cygni

The King of Daughton dies, without any heirs. According to local law, the law then passes to a blood relative, no matter how distant. Uprooted from his home, Dean Winchester is given a crown and the task of finding a bride. He must marry before he comes of age. Dean is on the eve of his 21st birthday, yet he cannot find love among the many that try to win his favor. Only when Dean meets a stranger, an enigmatic young man in the woods, is his heart touched. He falls for the mysterious Castiel, unaware of his terrible secret—that Castiel and his fellows have been cursed for years—cursed to transform into swans during the day, only returning to human form at night. And according to legend, the spell can only be broken with a proclamation of eternal fidelity. A proclamation of love.