Rance II – Hangyaku no Shoujo-tachi –

Ten years ago in the town of Custom, four young girls were appointed as disciples to old mage Ragithis. They were to be raised as mages and then to work as town's guardians. But after the ten years of training due to unknown reason girls rebelled against Ragithis and killed him. During the harsh magic battle a whole town were entombed in the underground and a barrier spell was put on town preventing everyone from going out.
This is when Rance appears with Sill on his side, and taking a job of defeating magician maidens and saving the town. But after some research he finds out that actual evil guy was Ragithis himself, who put four magical rings on four maidens. That rings are giving a great boost of magical power to it's bearers, but also able to steal magic power from virgin sorceresses and after collecting magic power of 40 maidens anyone could use these rings to obtain unlimited power. The leader of girls, Masou Shizuka, figured out Ragithis's plan and rebelled against him but it was too late – cursed rings could not be removed and the curse started to dive maidens crazy. Now they are sheltered in the labirinth beneath the town abducting young girls from the town and killing everyone who tried to go into the labirinth and stop them. It seems that the only way to release them from the curse of the rings is to steal their virginity from them, and the only man up to this job is Rance.

[Taken from the AliceSoft Wikia]