Somewhere within the files that load when you load the game on your machine is one that states "Your character is consummed by the wrath of the Mavin" (or something close to that). If it were not for that file, you could argue that it was just a myth but it seems clear that originally they had intented to have players keep their oath.
Omitting this from the game has added a lot to the flexibility of the game, allowing players to play good or evil as they choose so it can be argued that it was a good move to leave it out of the game as released.
Wyv