Ninja Gaiden Shadow Trivia (Game Boy)
And in addition to Zohvi's entry, this also explains why "Shadow Warriors" doesn't have an original soundtrack, but rather are made of - excellently - converted songs from the NES series.
This game was originally planned to be released by Natsume as an adaptation of "Shadow of the Ninja". Tecmo acquired the rights to it and after some cosmetic changes released it internationally as a prequel to it's long-running Ninja Gaiden series (that takes place 3 years PRIOR to the original game) as "Ninja Gaiden: Shadow". The (later) European release, as with all other Tecmo-released "Ninja Gaiden" games, renamed the game "Shadow Warriors" because, at the time, "ninja" were considered a taboo subject there.