The story behind Diablo is quite deep and detailed. It’s far from your traditional, cookie cutter stories. What’s interesting is that some of the Diablo 2 characters and Diablo 2 items from the first and second Diablo game are still related to the characters and events that transpired in the third game.
Some of the Lores and stories that is present in Diablo 3 (coming from Diablo 2):
• Barbarian – with the destruction of the Worldstone and Mt. Arreat, he vanquishes hordes of demons and saves his tribe from invasion, at the cost of disarray. Their tribes are now either wanderers or returned to their primal and primitive nature, corrupted by demonic powers and forces.
• Amazon – the matriarchal society of warriors from Skovos. Their race began with a Nephalem and woman named Askarra. From there the Amazon society began. The Amazon warrior who lent a hand in slaying Baal does not have accurate records regarding the aftermath or current whereabouts. It is certain though that they would be a part of future stories as they have possession of the Sightless Eye, the mirror that can communicate with the Heavens and show a glimpse of the future.
• Sorceress – her efforts was crucial with the war against Baal. Countless demons have been scorched or frozen eternally thanks to her spells. The Wizard for Diablo 3 is a rogue spell caster and not related though to the Sorceress in Diablo 2 which followed strict disciplines and remained loyal to her order.
• Assassin – their secret clan began with the Vizjerei mages, the brothers Bartuc and Horazon. Bartuc utilized dangerous magic and Horazon opposed his brother’s tactics. The disagreement led to a war, and levelled their city. The survivors then formed Viz-Jaq'taar, or Assassins, whose primary task is to slay any corrupt mage or Vizjerei, as Bartuc’s return lies imminent within the horizon. Their participation with the war against Baal was unexpected but nevertheless invaluable.
• Druid – after the destruction of the Worldstone, the Druid returned to Scosglen, where the remaining Druids live and thrive. It is rumored that the Barbarians led by Bul-Kathos were originally Druids, prior to their departure. They may return to lend a hand to their brothers but other future events remain unknown.
• Necromancer – the Priests of Rathma, dedicated to the fragile Balance, of life and death, order and chaos. Their vigilance with the Balance kept the demonic invasion at bay. The Necromancer that participated with the war against Baal is the mentor of the Necromancer that gives a quest in Act II the Desert.
• Tyrael, Archangel of Justice – with Mephisto and Baal leading the war, Tyrael mustered his strength to transport the heroes to Harrogath, where they could attack Mt. Arreat and finally face Baal. The heroes succeeded and defeated Baal, but Tyrael’s hardships only just began as fellow Angels opposed his wish to assist the humans from the incoming onslaught of demons. He stripped himself of his wings and became mortal, where he fell to the Sanctuary as a falling star. From this point he assists the heroes throughout the game and even brings them to the Heavens.
As you can see, the stories are weaved beautifully between each character. They all have their own reasons regarding their participation with the war against demons.