A series of four eroge: Izumo, Izumo 2, Izumo 3, and Izumo Zero.

Izumo (1) has been turned into a hentai OAV series, while the "Izumo 2" was adapted as the anime TV series "Izumo: Takeki Tsurugi no Senki."
"VNDB":http://vndb.org/v618