 is a Japanese visual novel. It was developed by Type-Moon. At first, it was released as an adult game for Windows. There is a manga series by Datto Nishiwaki based on this visual novel. It was published in Kadokawa Shoten's manga magazine "Shōnen Ace". A 24-episode anime series was created by Studio Deen. It aired in Japan between January and June 2006. A second anime television series, "" was produced by Ufotable. It aired between October 2014 and June 2015.