Diablo Immortal is somewhere in the middle of the Diablo story timeline. The new game is therefore both a sequel and a prequel.
It remains to be known whether the events of Diablo Immortal will continue to affect the Diablo 4 setting in any way. They are understandably still important enough to affect the main timeline anyway.
The following walkthrough will explain where and how Diablo Immortal is located in the timeline.
Table of contents 1) Understanding Diablo Immortal Timeline 1.1) What happened in Diablo 1 1.2) What happened in Diablo 2 1.3) What happened in Diablo Immortal 1.4) What happened in Diablo 3 1.5) What happened in Diablo 4
Understanding Diablo Immortal Timeline
Diablo Immortal takes place between the events of Diablo 2 and Diablo 3. It acts as a bridge to bridge the two-decade gap in the timeline between the two games.
According to available lore, Diablo Immortal takes place only five years after the events of Diablo 2 and 15 years before the events of Diablo 3.
It therefore becomes important to first recap what happened in previous Diablo games and how Diablo Immortal sets up the events of Diablo 3.
What happened in Diablo 1
Diablo's first game had a relatively simple storyline. Following a demonic invasion by Tristram, an unknown hero nicknamed the Stranger (and later the Dark Wanderer in Diablo 2) stepped forward to defeat Diablo, the Lord of Dread and one of the three Prime Evils.
In the Diablo games to follow, the Stranger is revealed to be Prince Aidan, the eldest son of King Leoric who had been driven mad by Diablo.
Aidan, however, only had one way to finally defeat the Lord of Dread. After containing Diablo inside a Soul Stone, he dipped the Soul Stone into his forehead in an attempt to contain Diablo and prevent his essence from escaping from his prison.
What happened in Diablo 2
Diablo 2 is set right after the events of the first Diablo game. The Dreadlord was simply too powerful. From within the Soulstone, Diablo was able to possess and control Prince Aidan to free his brothers and two other Prime Evils: Mephisto, the Lord of Hatred; and Baal, the Lord of Destruction.
Prince Aidan, the hero of the first Diablo game, became the Dark Wanderer in Diablo 2 due to his corrupt state.
Thankfully, a new hero has stepped forward to defeat the Dark Wanderer and the First Evils. However, not before Baal managed to corrupt the Worldstone, an ancient object that holds the power of creation and is believed to have created the Shrine itself.
In its corrupt state, the Worldstone threatened to corrupt and destroy all humanity. The threat forced the archangel Tyrael to descend from the skies to destroy the Worldstone in a massive explosion.
What happened in Diablo Immortal
Diablo Immortal has just been released and its storyline has yet to be fully revealed. The tradition itself remains to be updated.
What is known, however, is that the explosion at the end of Diablo 2 scattered fragments of the corrupted Worldstone around the world. The peace that was won at the end of Diablo 2 was therefore short-lived.
Fragments of the corrupted Worldstone have created a new demonic threat. There are cultists who try to find and exploit the fragments to open a door for the return of Diablo.
As Blizzard Entertainment says, these fragments are like pieces of uranium. Their radioactivity is not only fueling the demons to become more powerful, it is also slowly corrupting the lands of Sanctuary.
Diablo Immortal is therefore all about containing the fragments of the corrupted Worldstone.
What happened in Diablo 3
Sanctuary was somehow saved at the end of Diablo Immortal, and humanity enjoyed a long era of peace. That peace was then broken when the archangel Tyrael fell from the heavens to warn of the arrival of two of the four lesser evils: Belial, the Lord of lies; and Azmodan, the Lord of Sin.
Like the first two games of Diablo and Diablo Immortal, Diablo 3 introduced a new hero who would clash with the Lesser Evils. The hero would be a Nephalem, descendant of an angel and a demon.
Both Belial and Azmodan were eventually defeated by sealing them within the Black Soulstone (not to be confused with the corrupted Worldstone), but not before the Black Soulstone was used to resurrect Diablo.
Eventually, the Nephalem managed to defeat Diablo but was unsuccessful. The Black Soul Stone has been destroyed and thus the essence of Diablo as well as the other Evils within have all been released. Also, just like with the Worldstone, the fragments of the Black Soulstone are now scattered throughout the Sanctuary and threaten to release all Great Evils upon humanity once again.
What happens in Diablo 4
Diablo 4 is a long way to go and therefore no one really knows its storyline.
What is known is that Lilith, daughter of Mephisto and queen of the Succubi, has been released from her prison. Lilith is also the mother of the first Nephalem after her forbidden union with the archangel Inarius in tradition. She is believed to have been released by Rathma, a first generation Nephalem.
Fragments of the shattered soul stone are also likely to play a major role in the storyline with presumably a new hero taking on the cloak against the new threat to Sanctuary.