Escu:de - Yuusei Tenshi Primaveil Drei: Dark Mission

Lambert's little sister Nagisa has created the evil seed, which can be used to corrupt the Primaveil girls and make them fall to the path of obscenity.
This game mainly features sex training by Lambert in order to corrupt Primaveils by accumulating 3 types of crystals that correspond to the training you give them. Train them wisely, otherwise a bad ending awaits you.