Act

Diablo II is split up into Acts that handle both what type of level you will encounter but also showing clearly where you are in the story progression, and what quests are available to you. There are five acts (four without the expansion), each with individual tilesets.

PLEASE NOTE! SPOILERS!
This article contains a lot of spoilers for Diablo II.

Act I

The Rogue leader, Akara

The hero you are playing has just heard of the demonic troubles of Khanduras, and arrive at the Rogue Encampment, finding out the Sisters of the Sightless Eye in chaos, after having lost their Monastery to demons following the arrival of a Dark Wanderer.

After clearing a way to the Monastery, and dealing with the greater demon Andariel there, the road is finally free to travel East.

Act II

Jewel of the Desert

Reaching Lut Gholein by caravan, our hero helps solving a problem with a corrupted Horadrim mummy in the sewers of the city, and sets upon creating a Horadric Staff needed to find the Tomb of Tal Rasha, to stop the Dark Wanderer from freeing Baal, who is imprisoned there. It becomes apparent that there is a demonic invasion in the Palace, through which the hero finds the Arcane Sanctuary and later the sacred tomb where another Lesser Evil - Duriel - awaits.

Unfortunately too late to stop Baal being freed, the player at least encounter Tyrael, who sends you further East to Kurast, to stop Diablo from freeing Mephisto.

Act III

Central hub of the Docks

Arriving in Kurast Docks, it's apparent that the local jungle has been corrupted by demonic forces, encroaching on the once magnificent city. The player briefly sees the Dark Wanderer, but is unable to stop him. The hero sets out to travel to Travincal, and defeat Mephisto, who has taken control of the Zakarum Church. You are faced with the grisly task of collecting body parts from the last uncorrupted priest, Khalim in order to create an artefact capable of destroying the Compelling Orb to free the priesthood and reach Mephisto.

After much effort in the jungles of Kehjistan, the hero reaches Travincal, he destroys the orb and kills Mephisto, opening a portal to the Pandemonium Fortress.

Act IV

Last bastion of Heaven

Arriving at the last bastion of Heaven before Hell, the Pandemonium Fortress, Tyrael sends the hero out on the plains of Hell to destroy Mephisto's Soulstone, and to reach Diablo, destroying him as well.

Following a long journey through Hell, the hero reaches Diablo's Chaos Sanctuary, where he activates the five seals and summons Diablo, and a violent fight ensues. Defeating Diablo and destroying his Soulstone, the hero returns to Tyrael, who opens a portal to Harrogath to stop Baal from corrupting the Worldstone.

Act V

Barbarian fortification.

(Expansion only)

The hero is introduced to the Barbarian homelands, and in particular Harrogath. He is set upon breaking the demonic siege of the fortification, and to find the missing Anya, one of the last of the Elders.

Elder Nihlathak turns out to have betrayed Harrogath, and the one who kidnapped Anya. When he is dealt with, the Hero is faced with the Barbarian Ancients, who are guarding the Arreat Summit and the Worldstone Chamber where Baal has entered. Defeating the Ancients, entering the chamber and killing Baal, Tyrael announce that the Worldstone has been corrupted beyond repair by the demon, and sends the player away to destroy it.