Japanese Eroge developer Illusion's latest. I haven't gotten to play it yet but the wiki should give you some insight. As usual with these titles, it's censored and in Japanese but there are English and un-censor patches.

The wiki: http://hgames.wikia.com/wiki/Premium_Play_Darkness

The hongfire discussion: http://www.hongfire.com/forum/showthread.php/416693-ILLUSION-Premium-Play-Darkness-(%E3%83%97%E3%83%AC%E3%83%9F%E3%82%A2%E3%83%A0%E3%83%97%E3%83%AC%E3%82%A4%EF%BD%9E%E3%83%80%E3%83%BC%E3%82%AF%E3%83%8D%E3%82%B9%EF%BD%9E)-Character-amp-GamePlay-Discussion Sorry, lots of Japanese text.

I believe you can find a translation patch on Hongfire.

The translation patch isn't very helpful, or at least the two I found weren't. Sure, the text is -mostly- english, but it doesn't logically make sense 90% of the time. If there is a better translation, I'd love to see it.

English launcher and interface here:http://www.hongfire.com/forum/showthread.php/416587-ILLUSION-Premium-Play-Darkness-%28%E3%83%97%E3%83%AC%E3%83%9F%E3%82%A2%E3%83%A0%E3%83%97%E3%83%AC%E3%82%A4%EF%BD%9E%E3%83%80%E3%83%BC%E3%82%AF%E3%83%8D%E3%82%B9%EF%BD%9E%29-Discussion-and-FAQ?p=3398164#post3398164
Along with save file and other helpful patches.