Fairy King

Jack has moved to Hokkaido to get treatment for an illness, but he's lonely and unhappy. One night a man on a horse rides through his window to help him. For a reason he doesn't understand, Jack instantly knows the man's name and feels they've met somewhere before. The man, Cu Chulainn, tells Jack to open the moonlight shadow window, which leads him to Nymphidia, the world of Midsummer Night's Dream, whose inhabitants are waiting for a new Fairy King

Based on a manga from 1978 by Ryoko Yamagishi.