Kyomu Senshi Miroku
Japanese title: 虚無戦史ＭＩＲＯＫＵ
Type: OVA
Episodes: 6
Source: Manga
Genres: Action, Fantasy, Sci-Fi, Supernatural
Producers: J.C.Staff, animate Film
Duration: 25 min per ep
Rating: R+ - Mild Nudity

A psychic ninja fantasy suggesting that the assassins did not die out during the Sengoku era but burrowed under Japan, where they fought a new war against the alien occupants of a crashed spaceship that has lain undisturbed for several millennia.

(Source: The Anime Encyclopedia)