Lunacid

Long ago, a gigantic animal that emerged from the water doused the planet with poison. Nowadays, people who are thought undesirable are dumped into a large well that has turned into a den of disorder and sickness. After being dropped into the Great Well for reasons that are unknown to you, you come to in a moonlit underground environment. The only way out is to descend much farther and confront the elderly man sleeping below. There will be many creatures and secrets to find along the road.