Worldstone

The Worldstone, otherwise known as the "Eye of Anu", "Heart of Creation" and "Heart of the World" was a colossal, mountain-sized object of immense power. Sought by both Elder Angels and Demons, it would be used to create Sanctuary before being destroyed as a result of the Eternal Conflict. And yet for all its importance, few of Sanctuary's inhabitants are aware that it even existed.

Early History
The Worldstone originally lay in the realm of Pandemonium, the scar of the universe's violent birth from the battle between Anu and Tathamet. It was the foundation of all places and times, a nexus of realities. A foundation that the forces of Heaven and Hell sought control of. Oral history states that whichever side controlled the stone allowed it to alter reality and create life and worlds with barely any restriction. Some Elder Angels believed that the Worldstone was an actual eye belonging to Anu, ripped out of his socket during his epic battle with Tathamet. Indeed some even claim to have seen God missing with only one eye remaining.

Both Elder Angels and Demons created worlds that reflected their nature, but all of these worlds were inherently flawed, and were doomed to wither and die.

At some point, the archangel Tyrael ordered that a bastion be built around the stone, which would come to be known as the Pandemonium Fortress. The fortress changed hands many times over the course of the Eternal Conflict, and the possession of the Worldstone alternated in turn.

As the conflict dragged on, the Worldstone became 'poisoned' by Heaven and Hell according to Tyrael due to their desire to control its power, altering its original nature.

Sanctuary and the Sin War
Over the course of the Eternal Conflict, a number of angels and demons had grown tired of the endless war and desired a place away from the conflict. These renegades were led by the angel Inarius and the demoness Lilith. Either through strength of arms or misdirection, Inarius and his renegades gained access to the Worldstone. Altering its frequency or dimensional alignment, they used its power to conceal it from the angels and demons still fighting in the Eternal Conflict. Subsequently, the Worldstone was shifted into a pocket dimension. There, Inarius and his followers shaped a world around it. A world of refuge they would call Sanctuary. The forces of Heaven and Hell became aware of the Worldstone's disappearance, but had no idea where it was or who had taken it. Either way, far removed from the squabbles of his former allies and enemies, Inarius shaped Mount Arreat as a kind of protective shell around the Worldstone. From there, the rest of the world would be formed.

The Worldstone could be used to alter the abilities of certain individuals. Unknown to the renegades, Inarius tied the energies of the Worldstone into himself, enhancing his powers greatly. The Worldstone was further used to inhibit the powers of the nephalem—the offspring of both angels and demons and with the potential to become greater than both. Over time, as each generation of nephalem became weaker than their predecessors, the race that would be called humanity arose. However, the potential for Man to reclaim his former strength still existed, and during the last days of the Sin War, it was potential that began to be reclaimed. Lilith, having long since fallen out with Inarius, cast a spell on the Worldstone that would allow humanity's former power to return.

Events began to get out of hand, however. Uldyssian, the first to reclaim his nephalem birthright, came into contact with the Worldstone, changing the pattern of a small part of the artifact from five-sided to six-sided, thus changing its resonance. Unlike Lilith, he had changed the nature of the Worldstone itself, and the nephalem's powers began to return at an even faster rate. And Inarius could not undo the effects that Uldyssian's contact had wrought.

Uldyssian ultimately defeated Inarius and severed his connection with the Worldstone, but lost his own life as well. At the end of the Sin War, the Angiris Council and Mephisto agreed that Sanctuary and its inhabitants would be allowed continued existence, though one of the conditions was that humanity be taken 'back to the beginning.' This meant a second loss of their nephalem abilities, per another alteration of the Worldstone.

Worldstone's End
It is stated that when the world was young, the people of the Northern Steppes were given a sacred charge of protecting that which lay in the heart of Mount Arreat. These people were called Children of Bul-Kathos, a moniker that encompassed two distinct groups— the Barbarians, who remained in the steppes, and the Druids, who developed their own culture in Scosglen but vowed to return to the steppes at Uileloscadh Mor, the final battle between Heaven and Hell. For the Barbarians, the protection of the Worldstone was the cornerstone of their culture. Consequently, the Barbarians were quick to thwart any incursions into their lands and developed a reputation for savagery in the kingdoms to the south. Eventually, the moment came. Baal, Lord of Destruction, approached Mount Arreat at the head of a demonic army. His plan was to infuse his soulstone shard with the Worldstone that had spawned it. His shard pulsated with energies of destruction, and, if fused with the Worldstone, humanity would fall prey to the powers of destruction as well. The Barbarians fought to stem the tide, but were driven back to the slopes of Arreat and their last bastion of Harrogath. Baal's forces laid siege to the fortress while thanks to the treachery of Nihlathak, Baal was able to claim the Relic of the Ancients, allowing him passage up the slopes of the mountain.

Ultimately, Baal was defeated by a group of heroes that had previously defeated his brothers, Diablo and Mephisto. However, they were too late to prevent him from enacting his plan. Tyrael, coming to aid the heroes one last time, determined that the only recourse available was to destroy the Worldstone. After opening a portal for them, the angel hurtled El'druin into the stone, shattering it.

Legacy of Destruction
The destruction of the Worldstone had grave consequences. Baal's body and his demonic forces were destroyed, but the surrounding area was devastated. Mount Arreat was left a smoldering crater, and even hundreds of years later toxic clouds of ash and arcane dust hung over the surrounding area. What was once the Northern Steppes became known as the Dreadlands and the demonic corruption ran rampant, altering both man and beast. The Barbarians were left scattered. The heart of their culture had been destroyed, and many tribes fell into a state of regression.

The Worldstone's destruction also held grave implications for Sanctuary as a whole. Much of the mortal realm had been ruined due to the explosion, and the demonic corruption of the shards nearly turned the world into a barren wasteland. The Worldstone's dampening effect had prevented armies of Demons and Elder Angels manifesting en masse, but with that protection gone Sanctuary was open to invasion by the other supernatural races. But all was not lost for the nephalem. Without the Worldstone's dampening influence, humanity's inherent power was able to return in at least a select few.

When the Worldstone exploded, all the shards scattered across reality, each forming realms and universes of their own, laying the foundation for the other supernatural races like the Olympian gods, the Asgardians gods, etc. One such shard was even used to cut a small section of the Burning Hells off by the Devils and create the Underworld as their own home.