Storyline - WITH AN EXCITING ALTERNATE ENDING!
Here goes a quick storyline suggestion:
In Diablo II, while the three Prime Evils were wandering up on Earth, Azmodon and Belial were fighting far from Diablo's Chaos Sanctuary, on the opposite side of Hell. As they fought, the Archangel Tyrael, along with additional strength from Heaven, kept the hero moving on, destroying the evils in his/her way, slowing draining energies from Heaven. As soon as Diablo was slain and his soulstone shattered, Azmodon and Belial noticed and figured that Heaven was busy with banishing the Three Evils. They waited for the Worldstone to be destroyed, allowing one of them to launch a surprise attack on Heaven, since their eyes were blind from any new threat. Azmodon and Belial negotiated, and Belial was sent to Heaven with an army, taking over part of the drained kingdom. Azmodon continued his journey to the Abyss, when he discovered Diablo, Mephisto, and Baal's scattered souls. Even though Azmodon helped banish the Three earlier, he decided full control of Hell, Earth, and Heaven would result in bringing the Three back. Azmodon performed a ritual and brought each Prime Evil back within a Soulstone. The Prime Evils learned of Azmodon and Belial's plan, and Baal and Mephisto were sent to Earth to build demonic and undead forces for Belial to use, along with creating their own protective force. Diablo stayed in Hell, and built a fortress for himself and Azmodon to command their army and the armies on Earth.
Once Belial is slain in Heaven, his forces retreat back to Earth and help defend Mephisto and Baal. The new hero comes down to Earth and travels through the burnt and desolate lands, in search of Mephisto and Baal. Mephisto is found in the desert, raising undead minions from the tombs, while Baal is found in the forest and jungles, building siege equipment and barricades. Other lesser evils are resurrected by Diablo and Azmodon in the Abyss and they return to Earth, this time with a Soulstone. They are scattered throughout the land, and their armies plague the lands. The lesser evils also have the task of awaking the five dragons of the world.
Frostiliar : Dragon of Ice (Kilaronn Mountains)
Toxilias : Dragon of Poison (Aranoch Deserts)
Boltazaen : Dragon of Lightning (Jungles of Kehjistan)
Flambron : Dragon of Fire (Philios Island Volcano, Tempertop)
Hostiliar : Dragon of Magic (Ruins of Mephisto's old Temple)
Once the five dragons are slain, along with Mephisto, Baal, and the Lesser Evils, the remaining forces run back to Hell and the land of Earth begins to replenish - along with the energies of Heaven. The hero enters hell, and Heaven sends numerous Archangels to help with the final battle. Once Diablo and Azmodon are slain, the Soulstones the Hero has collected are all smashed. Now with all three Prime Evils AND Lesser Evils are sent to the abyss and are now helpless. No allies of theirs can reach the Abyss and perform a ritual strong enough enough to return then. End of Diablo series.
* ALTERNATE ENDING *
For those that would like to see the series go even longer to IV and V and VI, there needs to be an ending that allows the possible return of the Prime Evils.
During exploration of Hell after the enemies are all slain, the Archangel Jandrael is drawn to the compelling power of the Abyss, and finds that Tyrael is one of the souls struggling there. Without thinking, Jandrael performs the ritual to bring Tyrael back. Tyrael returns corrupt from the evil in the Abyss and slays Jandrael. Now alone, Tyrael can now bring back the three Prime Evils without anyone knowing. Diablo IV carries the story on. The ultimate evil force would be unleased on Earth. The unlucky 13:
Belial : The Lord of Lies
Azmodon : The Lord of Sin
Andariel : The Maiden of Anguish
Duriel : The Lord of Pain
Mephisto : The Lord of Hatred
Baal : The Lord of Destruction
Diablo : The Lord of Terror
Frostilair : The Dragon of Ice
Toxilias : The Dragon of Poison
Boltazaen : The Dragon of Lightning
Flambron : The Dragon of Fire
Hostiliar : The Dragon of Magic
Tyrael : The Fallen Angel