Questions to be asked.

[raveharu]

Ok, I am a little confused about the whole soulstone affair.

Facts known:
1) It seems Tyrael drove Baal's soulstone into Tal Rasha, and trapped him into the depths of Lut Gholein.

2) This also applies to the hero from Diablo, known as the Wanderer, after killing Diablo, he jammed the soulstone into his own forehead.

3) By jamming the soulstone into one's body, the essence of the Prime Evil contained will try to takeover the host's body.

4) So the reason why the soulstone were rammed into the body is to contain the Prime Evil's essence within one's body.

5) It also seems that the host will be boosted with powers from the Prime Evil's essence.

Questions:
1) How did Mephisto and Baal escape?

2) We have destroyed both Mepisto's and Diablo soulstone. Does that mean their essences have been released to the mortal world once again?

[KiLLJOi]

Didn't the soulstone provoke him into sticking it into his own head, that or I heard somewhere that it corrupts people into wanting more power and basically tricking them into doing this

something along those lines...

[Tsumaru]

For Baal, it was only a SHARD of the soulstone. Baal shattered it before they could trap him within. Thus it wouldn't be able to contain all of him. As such, Tal Rasha essentially sacrificed himself to kind of become an extension of the soulstone. Tyrael knew that the soulstone shard would be useless on its own, but thought Tal Rasha may be able to succeed when he suggested his plan. So he approved and they all buried him and bound him to that stone. And it actually looks like he was basically succeeding up until Marius set him free - although Baal was sorta getting control over Tal Rasha's body. He obviously didn't have the kind of influence Diablo did though.

As for Diablo and the Wanderer. The hero at the end of D1 realised that the solstone on its own wasn't keeping Diablo from influencing the world. So he thought that by shoving it into his head, he could use his own strength in addition to the soulstone to suppress Diablo. Deckard Cain remarks that he is "arrogant" and does not believe it to be possible. Turns out he was right, because Diablo ends up possessing him. Tyrael would not have been aware of any of this happening, I assume, and the hero from D1 acted on his own judgement. Presumably it was too late before Tyrael knew what had happened to do anything about it.

[raveharu]

For Baal, it was only a SHARD of the soulstone. Baal shattered it before they could trap him within. Thus it wouldn't be able to contain all of him. As such, Tal Rasha essentially sacrificed himself to kind of become an extension of the soulstone. Tyrael knew that the soulstone shard would be useless on its own, but thought Tal Rasha may be able to succeed when he suggested his plan. So he approved and they all buried him and bound him to that stone. And it actually looks like he was basically succeeding up until Marius set him free - although Baal was sorta getting control over Tal Rasha's body. He obviously didn't have the kind of influence Diablo did though.

As for Diablo and the Wanderer. The hero at the end of D1 realised that the solstone on its own wasn't keeping Diablo from influencing the world. So he thought that by shoving it into his head, he could use his own strength in addition to the soulstone to suppress Diablo. Deckard Cain remarks that he is "arrogant" and does not believe it to be possible. Turns out he was right, because Diablo ends up possessing him. Tyrael would not have been aware of any of this happening, I assume, and the hero from D1 acted on his own judgement. Presumably it was too late before Tyrael knew what had happened to do anything about it.

Ah, so you mean this Wanderer, who already has been possessed by Diablo, went in search for his brothers and released them?

[Tsumaru]

Yeah, the Wanderer is the guy from Diablo 1. He goes into Tristram, kills Diablo. Says to himself, "the soulstone isn't working, what if I shove it into my head and try to suppress Diablo myself?". He does this. He comes out of the cathedral, and all the people in Tristram love him. But he progressively becomes more brooding and dark over a period of weeks (or months, I forget). We assume this means that he is struggling with Diablo, though most in-game character people thought it was just being haunted by the horrors he witnessed. Eventually he is compelled to head East to release his brothers - Diablo has taken a fairly big control over him. By Act 2 when he releases Tal Rasha, there's basically nothing of his humanity left and he's pretty much Diablo in all but flesh. Then Act 3 he becomes completely Diablo, assuming his true form.

[Swiffer]

1) How did Mephisto and Baal escape?

Both were freed by Diablo in the course of Diablo 2.

2) We have destroyed both Mepisto's and Diablo soulstone. Does that mean their essences have been released to the mortal world once again?

2 trains of thought on this.

The first is that yes, their spirits have been released from captivity within the soulstones, the second is that they have been destroyed, though the cinematic would seem to suggest that at the least Diablo is still floating around out there.

[DBaron]

My understanding is that meph was already free, and pretty much had free reign of kurast (the whole corrupting of the council, compelling orb, etc). I guess he was just sitting around waiting for Diablo to pick up Baal, or spent his time making that portal.

[Tsumaru]

Ah, yes, you're quite right. Mephisto had basically done what Diablo did in D1. He'd reached out from the soulstone and started corrupting people. I'm not sure what the timeline is of whether he was before Diablo, after Diablo, or around the same time or not. But anyway, Diablo himself didn't know that Mephisto was free either, I don't think. So he would have had, in his mind, the plan to go rescue both Baal and him.

[Thirty-Thirty]

The timeline about Meph is an interesting question. We have Meshif, though. On leaving Act II:
Jerhyn tells me I should take you east to Kurast. I haven't been there for several years, but rumor has it that things are pretty grim.
and arriving in Act III:
Well, I gave you my word, and brought you here as promised. But by all that's still holy, I wish I'd never returned to this accursed place. This fetid jungle can't be the fair Kurast I left behind.
So it looks like at most a few years. To be really picky, we don't know how long a year is, but human lives seem to have the same duration, so probably about an Earth year. (Yes, I'm overly fussy about the details.) Since the rumours are that its grim, I was thinking Meph's only been around a short while. But...
Mephisto used this device to control the Zakarum Priests and their followers. The Orb can only be destroyed with an ancient flail imbued with the spirit of the one incorruptible priest.

Soon after his imprisonment, Mephisto worked his evil corruption on the Zakarum priesthood. All were turned to his dark ways, save one - Khalim, the Que-Hegan of the High Council.

Mephisto directed the other Council priests to slay and dismember Khalim and then scatter his remains across the Kingdom. The Priest Sankekur succeeded Khalim as Que-Hegan, eventually becoming the embodiment of Mephisto here on the mortal plane.
He says this in Act III, so somehow Cain already knows that Mephisto is free, embodied by Sankekur, in the depths of the Durance. Granted, Meph is still trapped inside by the Compelling Orb, but given that his minions scattered Khalim's parts, couldn't they just go and find them again to destroy the orb? That's going to need patching up, IMO.

Cain might know because he's Horadrim. If you can explain why no-one bothered to shatter the Orb, then I guess no-one knows that Meph is down there. They'd just see the Zakarum going bananas.

[Tsumaru]

Hmm... that's an interesting point. Cain seems to know a fair bit about Mephisto considering he seemed to know nothing at all about Diablo originally (going by the journal, and from what I can remember of D1).

[Millionflame]

Well, Meph is the oldest of the brothers, so it makes sense that he'd be the one the Horadrim had the most experience with.

[Alkey]

I thought the orb was keeping you out, not meph in. I also think Cain would know where mephistos stone was laid to rest, in Kurast, and it was the only thing that would explain what had happened to Kurast. As far as the flail and the orb, maybe he did some detective work while he was there and figured it out for the hero.

[DBaron]

I thought the orb was keeping you out, not meph in. I also think Cain would know where mephistos stone was laid to rest, in Kurast, and it was the only thing that would explain what had happened to Kurast. As far as the flail and the orb, maybe he did some detective work while he was there and figured it out for the hero.

Yeah, thats the way I remember it too. The compelling orb was created by Meph to help control the zakarum. And Cain probably figured out what was going once he got to Kurast.