Well, seems to me the Worldstone was destroyed. I figure the Worldstone's job was to keep the realms of our world and the "Demon World" seperate. As soon as the Worlstone was destroyed, the barrier between the two worlds broke down, and monsters started pouring right past Tyrael and up to Mount Arreat. The Prime Evils would, of course, be accompaning these lesser demons through the barrier, in order to take total control of our world.
With no Worldstone to keep the two worlds divided, the physical properties of the Demon World would start melding with ours, turning trees to dead withered staffs of wood, burning the ground and wildlife, turning the sky jet black and blood red. All this destruction would feed Baal. All the death in the world would feed Diablo. And the world's hatred of what is happening would feed Mephisto.
With no Worldstone to prevent the power of the Demon World from fueling the Prime Evils, it would be a very simple task for them to turn Tyrael, just as Izual was turned before him. The Prime Evils would then turn their attention to Harrogath, where the last member of the Horadrim, Cain, and the hero who beat them back, you, are located. Their demons would set siege to the town, attacking relentlessly to destroy it.
Our hero would start in Harrogath, where Tyrael's portal last sent him/her, and have to fight back up the charred and broken landscape to the peak of Mount Arreat. There (s)he would finally get a chance to see the full extent of the changing landscape. The earth dry and cracked, the rivers turned to lava and blood, the once healthy trees now no more than dry withered sticks. And for some reason, no sign at all of the Prime Evils.
Our hero would return to Harrogath to seek council with Cain, who will tell of a once sacred place where all life is worshipped, and where great mystical powers are located. This will surely be where the Prime Evils are going, for with such power they would truly be unstoppable. Cain will then reveal his true power, hitherto unknown to anyone, and transport him and the hero to this place.
The Prime Evils, however, will interfere with the spell, and instead direct the spell toward a waiting horde of monsters. Spent from the casting of the spell, Cain will have no energy to defend himself, and will fall, in an elegantly cinematic cut-scene.
The hero will become filled with rage, and begin to massacre all creatures in range of sight, cutting a bloody path across the land, fueled only by hate. The land will become scarred with the blood of devils and demons, piling up so high as to change the shape of the horizon. The hatred of our hero will draw the attention of Mephisto, who will send the familiar face of Tyrael to meet with the hero halfway.
Tyrael will make kind words to the hero, who does not know the fallen state of the ArchAngel. Tyrael will then present the hero with a white Holy Sword, designed to facilitate the killing of demons. Over the course of the conversation, Tyrael will make a slip-up and tip off to the hero that he's no longer the ArchAngel he once was. Tyrael will then attack the hero, who will thrust the sword into Tyrael's chest. The sword, purified by the strength of the hero, actually will kill the now-demon Tyrael, and destroy itself in the process. Tyrael's death will take the form of a slightly more disturbing, but just as cinematic, cut-scene.
The hero will then proceed North, for that is where Cain said the holy place was. Still fuelled by hatred, even more now that Tyrael has been turned and killed, the swath of death will be vast. Mephisto, still drawn by the hate of the hero and now certain Tyrael has failed in his task, will come.
The hero will encounter Mephisto, and a great battle will begin. The hero will, inevitably, be beaten back, but not destroyed. Mephisto wants the hero's hate to live long and healthy, as it is such a good source of fuel for him. The hero will then retreat, beaten and broken, to solitude, in order to reflect on what has just been endured. (S)he will then realize what must be done, and return to Mephisto. When confronted, the hero will release all hate, and embrace the power of love. Love for fellow man, love for Cain, love for Tyrael who must by now be cleansed and residing in Heaven. Powered by the unstoppable force of love, the hero will combat the Lord of Hatred with peace. In a cinematic cut-scene involving lots of white light, Mephisto will be destoyed from the inside out, because the power of Hate cannot stand up to the power of Love.
The journey will then continue Northward, where Baal has sensed the genuine destruction of his brother Mephisto. The bloody path will still be cut by the hero, but no longer fuelled by hatred. The cause is now to reach the holy place, because only the purest light can defeat the purest evil of Diablo and Baal.
The hero will encounter Baal at the base of a great mountain. The battle will again be fierce, and yet again the hero will discover he is no match for an unrestricted Lord of Destruction. The retreat will be made, and in the retreat the hero will discover a small outpost village that has not been corrupted by the invading Demon World.
The town priest will reveal the existence of a minor magical barrier surrounding the town, through which Evil cannot pass. However, the barrier is losing power, and the priests are running out of energy to charge it with. In town, the hero will find a pregnant woman who also has a baby, and lead her out of town. The forces of Baal will have descended upon the town, and the hero must fight now not just to reach and destroy Baal, but to protect the woman and child. The path will be cut, and Baal will be reached. The hero will present the mother and baby to Baal, showing the Lord of Destruction the ultimate act of Creation. When confronted with this, Baal will not be able to endure, and will fall in a cut-scene involving lots of squishy guts and gory blood.
The hero will return to the small village to rest for the night, before continuing North to encounter Diablo. This will be chronicled in the expansion.