Yousei Ou
English title: Fairy King
Japanese title: 妖精王
Alternative titles: Youseiou
Type: OVA
Source: Manga
Episodes: 1
Status: Finished Airing
Aired: Dec 21, 1988
Duration: 1 hr
Rating: R+ - Mild Nudity
Studios: Madhouse
Producers: Kadokawa Shoten
Genres: Fantasy
Themes: Isekai
Demographics: Shoujo

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. 

(Source: ANN)