Centuries ago the evil sorcerer Azazabus built a home for himself under Mount Dread. All kinds of evil monsters came to help Azazabus. He paid some well, and for others there were other evil rewards for helping him. For years his monsters raided the farms nearby while the wizard worked on terrible experiments in his underground laboratory.
When the sorcerer finally disappeared, the monsters stayed and made the place their own. The dungeon is a good place for them to live and is easy to defend. Monsters have lived there now for over a hundred years.
This is a third-party system conversion of Under Mount Dread (a scenario bundled with First Quest) to Fantasy Grounds virtual tabletop.