Torment does the same thing. If I remember correctly, if you go to the undead sanctum first, you get a mission to kill the Many-As-One, I believe, whom is the rat king/god, and it's extremely hard to kill him at low levels.
But the rat god in Fallout 2 made no sense. It was just a big rat that was named. They didn't even give the thing a background or anything The rat infestation and the warrens with the semi-sentient big monstrosity felt more like a Hammer horror allusion than something that would have fit Fallout.
Torment had the advantage that Many as One was consistent with the world and an interesting character.
Edit: So, I started a IWD 1+How+TotL campaign with four multiclass chars: a half-elf fighter/druid and a cleric/mage, a gnome fighter/thief(absolutely awesome) and an elf fighter/mage.
Man, I'm excited: a new IE game!