Knights of Xentar is actually a port of Dragon Knight 3, a japanese role playing game and the third in the series (which now has 5 titles) that pretty much invented the H-RPG subgenre. For some reason Megatech couldn't get the rights to distribute the game with it's original name so they had to change it for legal reasons (a common happening in the world of animé/manga translations), Megatech included their legal woes in the game however, and you can read all about it in the Hentai scene with the princess where she starts blabbling about how sad it is when legal crap gets in the way of good entertainment (quite a surreal experience actually, but I don't think anyone bothered reading anything while the princess did her thing).
Aside from that, the translation is supposed to be quite unfaithful to the original and the NR-13 rated version was created by Megatech by editing some pictures and removing some stuff. The only japanese version is the full NR-18 release.
Furthermore, the Dragon Knight series spawned an animé OAV series as well as a manga, and not the other way around.
Near the end, one of the creatures you encounter is a "Tetsuo". This misshapen baby-like blob is taken directly from the animated classic "Akira".