If youre playing on Steam, keep in mind theres regular Overlord 1 and Overlord: Raising Hell. You only want to play Raising Hell, which includes the expansion pack. That expansion pack adds a section to the end of EACH story chapter, rather than being self contained.

If you play Overlord 1 and then try to play Raising Hell, you’ll be faced with a complete play through of the whole game just to see the expansion content. Theres no need to own base Overlord 1, or to play it if you have it.