According to the artist, the text in seal script is supposed to be read as "布都御魂剣" The title of the picture seems to be the pronounciation of the characters: "Futsu-no-mitama-no-tsurugi". A tag shared by this picture and post #994760 is "剣を祭る巫女" (Sword worshipping Miko) and both involves Futo holding ancient swords. It is safe to say that this is likely Futo's. Entering the characters above gives me this wikipedia article: http://ja.wikipedia.org/wiki/%E5%B8%83%E9%83%BD%E5%BE%A1%E9%AD%82
