Overlord is kind of an RPG type game, where you roam around as, well and Overlord. You control a host of minions, and you can run around causing havoc, or help people. Your character looks suprisingly similar to Sauron though... Overall it's a fun game, it has online co-op and versus aswell.
This is a .daa file (similar to an .iso) So you'll need something that can mount images, like poweriso or daemon tools.
After you update or whatever, you'll have to get a nocd patch. gamecopyworld.com has a good one. Have fun!