User blog comment:Breywood/Some Minor Detail in Diablo III/@comment-84.119.46.191-20120530204943

You are probably missing the point that basically everything is possible with these stones, the autors of the stories make them up, expand them or revise them as they go along. I seriously doubt that at the time D1 was being made they had the idea of a worldstone/black soulstone.

In the D1 manual it is said that the soulstones, once activated, create a spirutual vacuum that sucks in the the prime evil. Keep in mind that on the mortal realm neither of the three has a body but needs one of a mortal as a host. They are bodyless in the mortal realm because they have been banished there by the 4 lesser evils after their defeat. Duriel and Andariel in D2 have not possesed a human host as far as we know because they had come here on their own.

Lazarus sets Diablo free by shattering the soulstone. Obviously the abiliy to keep the demons locked is dependent on the size of the stones since Baal's stone was broken/chipped in battle it was too weak to contain the demon effectively. Hence Tal Rashh's sacrifice. When Lazarus threw the stone against the ground it broke in pieces and the at this point already also corrupted stone (fragements) was no longer able to hold Diablo. That is why the hero in D1 drives the (supposedly biggest) shard of the stone into his forehead. The does the same thing Tal Rasha does: He aids the the soul stone in trapping Diablo by constantly fighting the demon. Without this act, Diablo could have taken possession of any weak minded mortal in the surrounding easily.

The whole scheme with Izual is not really well explained. It seems that the plot of the three (or better yet Diablo's plot) revolves around corrupting the soulstones into channeling their spirit form into the black soul stone from D3 once they are defeated and their initial stones "destroyed". How Duriel and Andariel ended up in the black stone is unclear to me because they did not have stone to begin with. Maybe it has something to do with the worldstone, the black soulstone is the last piece of it after all (all soul stones are just fragments of the world stone). Or maybe they travelled into it by themselves deliberately once they were defeated and only a spirit - for that of course they would have to have been brought in on the schema some time after the dark exile.

Now, that crushing the soul stones results in destruction of the demon may be simply a wrog conclusion in game too, one that Tyrael makes also. The Seven can not be destroyed, they are reborn in hell just like the 5 of the angiris council are (their so-called "replacement" shows of at the crystal arch). All of them are parts of Anu or Tathamet respectively, none can be destroyed - it's much like the conservation of energy from physics. This of course is introduced D3 so it is a very unsatisfactory explanation for why Diablo and Mephisto didn't die after the destruction of their stones at the hellforge (again, I don't believe that they had he concept of a black stone figured out yet in 2000/D2). Baal's stone was never brought there as far as we know but the destruction of he world stone and the resulting blast may have destroyed it itself.

There seems to be a connection between the 3 initial stones, the world stone and the black stone. I assume hat once the red, blue, yellow stones and the world stone were destroyed they transfered their contents to the black stone. That was the nature of the "corruption" of the stones that the three were working on for about 200 years each and that Baal then continued with the world stone. Tyrael and anybody else seem to have missed that.