-Diablo working with Adria managed to absorb all the other Evils, become the Prime Evil and succesfully raided the High Heavens.
-By striking him down his soul along with the other evils was trapped within the Soulstone.
-Malthael saw this as an opportunity to wipe all demonic beings (humans too) from the face of existence, destroying most of mankind.
-Malthael fused with the souls when he shattered the soulstone.
-When we killed him, we destroyed them as well. However, when demons die they return to the Black Abyss where they are reincarnated.
In conclusion, the High Heavens are in the worse shape they have ever been, Sanctuary is mourning the countless dead and, finally, all the Great Evils are somewhere in Hell with fresh respawn timers.
At this point we have to remember the following:
-Heaven's power is limited whereas Hell's isn't.
-The very first being in all of creation, Anu, was the embodiment of everything, light and dark. When he got rid of all the evil from himself, it took form and became Tathamet, the Prime Evil. They fought until both fell and from the remains of each angels and demons came into being.
-Inarius, an archangel advisor to the Angiris council, serving under the command of Tyrael, has doubts about the Eternal Conflict. He gets caught during an attack and in captivity meets a demon who feels the same, Lilith. Together they gathered others who were fed up with the Eternal Conflict and using the Worldstone they escaped in the newly created world of Sanctuary.
-From the union of angels and demons a new race was born with the ability to surpass their ancestors' power; the Nephalem.
And this brings us to the interesting part. Throughout the games in the series, we have constantly tried to hold the demons from conquering the Sanctuary with the aid of angels (Tyrael) and their angel groupies (Horadrim, Cain was one). So far the story has shown us that angels=good and demons=bad which leads us to the conclusion that the eradication of all demons would be the ultimate goal for us.
However, this approach has made us overlook one thing, the balance. The first being in the universe was everything combined, all aspects of both good and evil. Inarius realized the futility of the Eternal Conflict because it can't have a winner. When there are no demons left there won't be any balance of dark and light in creation which can only lead to two possible directions. Life will cease to exist due to the absence of the dark or evil will manifest through others (what Inarius did is viewed as a sin by the angels) which ultimately means that the Eternal Conflict didn't actually end.
So we come to the point of realizing that our actions have no significant consequences in the overall picture of what's happening around us, the player character. But, there is a way to continue the story in a meaningful way.
In the next expansion we will try to venture into hell in order to either finish off the Evils or try and get Leah back. Up to this point anything can happen. During our journey in Hell we will come across Inarius, who will be tortured and deformed. He will explain to the hero why he abandonned his post and shed light on the futiity of the Conflict. The hero will then put Inarius out of his misery and continue to put an end the the war by creating a truce or possibly severing and cutting of any connections between heaven, hell and the sanctuary.
-Inarius might not want to go without a fight.
-Imperius will not just stand there and agree with the demons-are-friends-not-mindless-sadists campaign nor he will leave them be without pursuing their destruction.
-Diablo, since we are in hell, won't just give up on trying to gain dominion over Sanctuary. Also, since he absorbed the rest before he died, it could be that Tathamet will make an appearance instead of him.
-If Tathamet won't happen, Inarius is held by Mephisto so there could be some action from the other Prime Evils as well or at least their lieutenants.
-Tyrael forgets to eat again and this makes you mad. I mean REALLY mad. So you decide to do him a favor and rid him of his hunger forever...
Areas: Most of the Burning Hells, Heaven, the Black Abyss
The story can conclude by either:
-Creating barriers or severing the connection between the realms
-By defeating Imperius and the Evils, others will rise to power and they may come to a truce.
TL;DR: Pew pew pew, many dead, more pew pew, more dead, truce, no dead. The end.
(I posted this on both EU and US forums. I read all the rules and guidelines but can't find if this is allowed or not.)