How were the Prime Evils exiled to sanctuary during the dark exile? Specifically.

[Duncanîdaho]

I've always loved Diablo lore, and as such have followed it since back when Diablo 1 came out. There is one thing that doesn't quite make sense to me.

When the Lesser evils banned together to exile the Prime Evils, how did they get them to Sanctuary? Also, how does Tyreal get to Sanctuary? In all the lore I know of, the Worldstone served as a barrier and cloaking device for the world against heaven and hell. The world is named Sanctuary because of this. It is a sanctuary from heaven and hells war.

How were these beings able to bypass the blockade suddenly? I know heaven and hell eventually found Sanctuary's location but couldn't send their armies. The reason Baal corrupts the worldstone is so that it allows hells forces to flow freely into Sanctuary and not heavens. This is why Tyreal destroys it in D2:LOD, with other possible motives as well.

The only explanation I can think of is that the barrier was only strong enough for common and lesser evils and angels? The Prime evils, once the world was found, were strong enough to bypass this barrier together. With the prime evils on the other side now, it was easier for them to bring common demons over(the ones we fight), but still not everything fully. The Angels then do a similar thing to get Tyreal over maybe.

Relevant reading:
http://diablo.wikia.com/wiki/Dark_Exile
http://diablo.wikia.com/wiki/Worldstone

I'm curious to other people's thoughts on how they came over. Or if there is a specific explanation of it out there somewhere that I'm forgetting or haven't scene.

[Digits]

I've never seen it mentioned that the Worldstone actually served as a barrier from heaven or hell. Doesn't mention that in the wiki entry on the stone either. I think it only hid Sanctuary from both sides, unless I missed something. They did retcon a few things, so it's possible.

I'd say the demons were too busy fighting amongst themselves and the angels voted against wiping humanity out and left them alone. Tyrael is the only one who cares, the rest of them still sees humanity as an abomination and won't bother with them.

[Aquamonkey]

Initially, the World Stone was only like a cloaking device. Eventually, both sides found out about Sanctuary. There was somewhat of a truce between Mephisto and High Heavens to leave Sanctuary alone. Nothing really barred either side from entering/leaving. The World Stone was also changed to dampen the powers of the Nephalem.

[Duskmourne]

The thing with Diablo Lore is that it makes sense so long as you don't focus on it or think about it too much. When you start asking real questions about it, it quickly grows silly.