Based on the Toshio Maeda manga/video series I don't know which for sure but, from the synopsis, I'm guessing probably one of the later videos. If you've got a better idea please email. Programmed by Dez Climax who later went on to release a similar PC98 game based on another Maeda work La Blue Girl.
Anyone familiar with the concept of Adult PC games will have a pretty good idea of how this game works. For those who don't; each scene is rendered as a static bitmap and the gameplay (what there is of it) involves repeatedly clicking actions with the mouse (look, think, talk etc) from a menu, occasionally causing new actions to appear. Eventually all of the actions are exhausted at which point the scene changes, by way of a dissolve, and you do it all over again. Outside of blinking and moving lips there is very little in the way of animation.
The storyline concerns a girl who has terrible memories of her friends being attacked and killed by monsters. One day her life gets worse when a gang of randy fellow students attack her. Fortunately the gang is fought off but by a demon tail which unexpectedly sprouts from an unmentionable place on the girl's body. Understandably distressed by this new appendage she seeks help from her new pals who take her to visit a young woman priest hoping she will be able to rid her of this unusual problem. As you may have guessed things get worse before they get better as the group are under repeated attack from sex-crazed, multi-tentacled nasties.
It's hard to tell which role the player is supposed to be taking; the lead girl or an unseen male, it's never really clear but that's not point of this game. It doesn't take a genius to work out that the main purpose of Demon Beast Invasion is nudity and it is almost nonstop. The game opens with the player attempting to persuade one of the lecturers to remove her underwear and it keeps going from there with barely a scene going by that doesn't involve naked ladies!
Whereas La Blue Girl featured several 'spot-clicking' exercises and an action sequence to break up the game a little Demon Beast features none of this variation making it not only dull but extremely easy to complete since there is no way to lose! The roughly one hundred or so scenes took around an hour to click past. Not a lot of game for you money! Once it's finished you can re-live some of your favourite images and scenes from the title screen not to mention listen to your favourite tunes. Speaking of which the music is actually quite enjoyable. There are around ten tracks in all including one for general ambiance and three for subtle degrees of 'Sexy Time'.
To sum up, even if you are into naughty pictures this game has very little to offer. With less than an hour of game time it's over on your first try. There is no replay value (unless you're a masochist) and the 'click, click, click...' gameplay get's dull after battling your way through the first screen.