1. Tyrael commented that "for a time, it seemed, we would know peace" which assumes that some time passed after Diablo was slain on the Silver Spire. The Black Soulstone remained in the High Heavens.
2. Tyrael goes on to say "but it was not to be. Diablo's essence lingers in the Black Soulstone" and as this is being said there's a black "smoke" or "shadow emanating from it. "I can not destroy the stone, nor allow its evil to remain within the Heavens." While he is saying this there is shading going on in the image of him, growing darker, and then it shows the other angels of the Angiris Council. Imperius is pointing and the other angels are all armed and seem disturbed. "And so it must be hidden even from the angels". I think this is suggesting that Diablo's evil was corrupting them. Just as Diablo was able to work Leoric's mind, as the Lord of Terror, even from within his own soulstone...his power was working their minds.
3. Tyrael takes the Black Soulstone back to Sanctuary where he recruits new Horadrim to assist him. He then finds a suitable hiding place. Judging by the images of the staircases winding downward it was a place deep down where he thought nobody would look. Somehow Malthael knows where to find the Black Soulstone. Perhaps this is a trait of his abilities as the aspect of wisdom? Only the Worldstone has proven to have the power sufficient to hide things from Malthael.
4. In the Gamespot Crusader game play video it shows the Naphalem approaching Westmarch when he meets the Horadrim Tyrael had sent to find him. The Nephalem then continues into the city through the sewers. Malthael seems to be turning people into reapers and blocking the Nephalem's path as he goes. So clearly Malthael is in Westmarch doing we're not told what. It would seem that perhaps Malthael is attacking the major cities of Sanctuary and bringing death to humanity. His motives are yet unknown.
So I'm thinking the Black Soulstone caused further corruption in Imperius. I believe he will play a part in RoS. Malthael wants to destroy all demonic beings, or at least that was his purpose at one point, so it stands to reason that is why he has taken the Black Soulstone (since all of the great evils are trapped within) and why he is attacking humanity. The BSS (because I'm tired of typing it) was designed to be able to contain limitless spirits so perhaps Malthael is putting all the spirits of the humans he has slain into the BSS. Perhaps Imperius joins him...given his obvious disdain for humanity?
Meanwhile the greater evils seek to escape the BSS. This would seem possible due to the scene in Act 3 when the 5 spirits that were contained in it attempted to escape and all the shadow creatures attack in the armory. Perhaps with all 7 greater evils it could be done? Perhaps Adria gets involved?