Well that would have worked wonderfully except:Quote from Magistrate
In regards to your last paragraph about Mephisto not knowing the location of the Worldstone: It could have been that they charged the humans of the time with hiding it, and then through centuries of myth and legend, it was forgotten, leaving the Barbarians only with the memory of a charge. *shrug* Humans have always been the neutral party that either side has wanted to sway (or not sway). Since they were not involved with the pact, they could hide it so neither side would know where it was.
A: How exactly do you move a 100 ft tall stone enrenched inside of a mountain?
B: And even if the humans, without powers, could have done that... they didn't. The stone has always been inside of Mount Arreat
I don't know about you, but I'd think Mephisto would assume the stone would still be inside Mount Arreat if he knew it was there to begin with. And he's not likely to suffer from amnesia either, so lo luck there.