Here is the story of Diablo from the begining

Alright our story begins at the beginning quite literally in the Universe of the Diablo games it starts in the beginning with the universe being created and at this time humans didn't exist the only inhabitants of the universe were the angels of the High Heavens and the Demons of the Burning Hells.

They had been fighting since the beginning until eventually one individual and angel grew tired of the constant battling and planning of the day to day conflicts and decided to leave this never-ending conflict and go his own way. The individual i am talking about is Inarius tactical advisor of the Angiris Council the ruling body of the High Heavens consisting of five archangels who govern and administrate the laws in Heaven. With each of the five archangels having a sphere of influence which denote which functions and duties that they preform in the High Heavens. The five archangels also embody an emotion from which they themselves are the embodiments of and usually act in accordance to what they personify. The five archangels are Tyrael the archangel of justice, Auriel the archangel of hope, Malthael the archangel of wisdom, Itherael the archangel of fate, and lastly Imperius the archangel of valor and the leader of the Angiris Council and commander of the armies of the High Heavens.

Now Inarius went to the High Heavens and tried to reason with the Angiris Council and try to dissuade them from further conflict with the Burning Hells.

So after his failed attempt at persuading the council to reasoning Inarius gathered like minded angels and demons to his cause and went far away from the unending conflict of the war. After doing this Inarius stole the ancient item the Eye of Anu and the Worldstone Inarius created Sanctuary the Earth of the Diablo Universe. This place Sanctuary was a safe haven for demons and angels to live in peace and harmony from the unending conflict and from this Inarius fell in love with a like minded demoness Lilith and from their love and union the Nephalem were created a hybrid of both species with the potential to be stronger than both individually

But after the Nephalem were created Inarius saw them as abominations and wanted to eradicate them while Lilith saw the potential in the new species to become stronger than both races together and wanted to use them from her own ends and so she hunted down and killed all the renegade angels and demons. After she committed this act Inarius banished her to the Void and set out to eradicate the Nephalem but he didn't have the power to do it so he used the Worldstone to dampen their powers. Even after all of that everything was peaceful and good until the Burning Hells discovered Sanctuary and realized the strategic value of the humans on Sanctuary so devised a subtle plan to get the humans over to their side by creating the Triune a cult that would bring the humans over to evil. However Inarius saw the Burning Hells intrusion and created a countermeasure to the Triune the Cathedral of Light who acted opposite to the Triune in bring humans to the light and not to the darkness.

However things changed with the return of Lilith who sought to bring the Nephalem under her control so by weakening the dampening effect on a simple farmer Uldyssian she manipulated him to bring down the Triune and Cathedral of Light. At first this worked for Lilith but after Uldyssian and his nephalem defeated Lucion and destroyed the Triune Lilith in the chaos of the situation lost control and was severely weakened by Uldyssian allowing Inarius to banish her back into the Void. After Lilith's banishment the Nephalem turned their attention to Inarius and the Cathedral of Light but as this was happening the Angiris Council discovered Sanctuary and convened with the Prime Evil Mephisto to decide humanities fate while through the voting the Council was torn but eventually it was voted for Sanctuary's continued existence after that decision was made Mephisto proposed a truce with the Angiris Council if they were to give him Inarius the forces of the Burning Hells would no longer mettle with the mortal realm so after this was agreed to it seemed that humans were now out of the Great Conflict but not all as it would seem

However the Three Prime Evils would not let go of Sanctuary and decided to act directly so with a fallen angel Izual they made an insidious plan to shatter the boundaries between the world of Sanctuary and the Burning Hells

But before i continue with my telling of the story of Diablo i am going to tell you about the opposing force to the Angiris Council and humanity as a whole and the commanding force of the Burning Hells the Three Prime Evils. The Prime Evils and also to a lesser extent the Lesser Evils are the ones who command and control the armies of the Burning Hells in their unending conflict with the High Heavens the Primes Evils are three powerful demons that are the strongest demons in Hell and therefore the ones who decide the Burning Hells direction in the conflict. The Prime Evils are not only just three of the strongest demons but brothers who each like the archangels of the Angiris Council have a sphere of influence that they personify and act on. The first of the Prime Evils and the youngest is Al'Diablos or Diablo the lord of terror is the most powerful of the Prime Evils and the Namesake of the Diablo series of games.

The second eldest of the three is Tor'Baalos or Baal the lord of destruction and is seen as the second most powerful of the Prime Evils. The third and eldest of the Prime Evils is Dul'Mephistos or Mephisto the Lord of Hatred he is seen as the smartest of the three but is the weakest of the brothers and the only one to have children Lucion and Lilith. Now the Prime Evils feigned holding back in the Great Conflict causing much displeasure with the Lesser Evils until eventually they staged a coup and banished the Prime Evils to Sanctuary in what would be called the Dark Exile. But this was to an extent part of their plan by being banished from hell and now in their spiritual forms they appeared as a lesser threat than they had been previously making the High Heavens jump on the chance to place them into soulstones to trap their spirits into the stones for the rest of time. So as the three were loose on Sanctuary and destroying and devouring the Archangel Tyrael made an order of mages called the Horadrim and under his help and guidance Tyrael interested them with the knowledge of the stones and how they could be used to trap the the three Primes.

So eventually the Prime Evils were captured one by one Mephisto was captured first and his stone was placed under the temple of light in Kurast were the order of the zakurum would watch for any sign of his escape. Baal was next were they cornered him in the desert of Aranoch were in the ensuing battle Baal shattered his stone and Tal'Rasha selflessly offered himself to contain Baal soulstone and be sealed into a tomb to wrestle with Baal for all eternity. Finally Diablo was captured in the Western lands and his stone was buried under the city Tristram's Cathedral and guarded by Horadrim seeing all the brothers captured and their stones hidden away the Horadrim thought there job done but slowly the Prime Evil began to twist and push the boundaries of the stones slowly corrupting them to their own needs. Mephisto was the first to get out and spread his evil throughout the Eastern lands Diablo was next possessing the hero who defeated him and from that he freed Baal and that lead the brothers to reunite under Travical and combine their demonic energies to open the infernal gate allowing Diablo to enter the Hells again.

As before the brothers were defeated but only Mephisto and Diablo were defeated and their stones destroyed but Baal remained hidden making an army of destruction to invade the holy barbarian mountain of Mt Arreat were the worldstone lay waiting for Baal to claim. So Baal accomplished his goal of corrupting the worldstone and Tyrael was forced to destroy it but the brothers plan was accomplished the barriers between the Hells and Sanctuary were removed. After the Worldstone was destroyed the inhabitants of Sanctuary braced for an invasion from the hells but it never came it remains to be seen what has happened to the conflict between Heaven and Hell but one thing was certain Sancutary's battle had just started but the Sin War was far from over.