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.