This is a single player RPG, and I believe the "expansion pack" has a completely different storyline. So it's kind of like Dungeon Siege 1 and Dungeon Siege: Legend of Aranna (Same engine, different story)
Gotcha. If you install it anytime soon, let me know if there's any extra DRM. I thought for sure I once read it had TAGES or something but I'm not seeing that on Steam.