The full story of Diablo: Timeline, lore, and history summarized

Diablo Lore: The Eternal Conflict

When Demons aren't trying to kill and overthrow eachother, they love fighting Angels. 

There is more to the Conflict than simple brawling however, the fight for control over the World Stone. The World Stone being the Eye of Anu at the centre of the Universe which has the ability to shape reality and create worlds and life.

While the Pandemonium Fortress was built to protect the World Stone, possession of the Fortress itself passed between Angels and Demons many times over during the Eternal Conflict. Whilst Angels were in possession of the World Stone they would create worlds of light and perfect order, but these worlds were also stagnant and devoid of life. Demons in possession of it made worlds of chaos and destruction, basically toddlers playrooms for evil. These worlds too were unsustainable.

Regardless the war raged on for an unspecified amount of time, but we figure a long time given the name Eternal. Worlds were endlessly created and destroyed by either side in a bid for control over the Heart of Creation.

Diablo Lore: The Prime Evils and Lesser Evils

Demons do not live by the same rules of order as Angels, they are far more selfish and hungry for power, but they do follow a basic hierarchy most of the time when they aren't infighting. The heads of Tathamet we discussed earlier are now the Great Evils and each rule over their own domain. Generally, these seven Great Evils rule their own area and demon armies, but their are exceptions to this rule and demons that refuse to join any particular faction.

The Three Prime Evils and most powerful demons in Hell are:

• Diablo - Lord of Terror
• Baal - Lord of Destruction
• Mephisto - Lord of Hatred 

While it's debatable who is the most powerful of the Three Prime Evils, Diablo is generally held as the most fearsome and unpredictable due to his untethered ambition and abilities of manipulation. Diablo is probably the most chaotic of the three and obsessed with having ultimate rule. As the youngest of the Three, he's a typical trouble-making little brother.

In contrast, Mephisto is the oldest of the Three, and the second being to have been created from the destruction of Anu and Tathamet (Malthael was the first). Mephisto is more rational and strategic, but also the most hateful, which being the Lord of Hatred comes with the territory. Mephisto is the father to Lilith and Lucion. 

Baal, the Lord of Destruction is the main antagonist of the Diablo 2 expansion of the same name. There has been conflicting information within the Diablo stories about if Baal or Diablo is the youngest but since the release of The Book of Cain, he is generally regarded as the middle sibling. You may hear the community mention Baal runs regularly as repeatedly killing Baal in Diablo 2 was the best way to level up characters and get loot.

The Lesser Evils are made up of:

• Azmodan - Lord of Sin
• Belial - Lord of Lies
• Andariel - Maiden of Anguish
• Duriel - Lord of Pain