"What would happen if you gave the Doom engine a massive upgrade, then used it to allow the player to wage a one-man insurgency on a fantastic Dungeons and Dragons plane stand-in?"
This game in a nutshell.
While suffering from the problem of "needs a base-game co-op mode", as well as a minor case of "press a button, something happens somewhere and you don't know what because the level design wasn't the best", the game was good enough to warrant Quake 2 aping off of it's "hub by hub progress" sensibilities.
To top it all off, this very easily ends up being the best showcase of the original idTech engine simply due to the sheer amount of modifications done to it. Atmosphere of this level wasn't accomplished again quite until Duke Nukem 3D.