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.
The hero you are playing has just heard of the demonic troubles of Rogue Encampment, finding out the in chaos, after having lost their Monastery to demons following the arrival of a ., and arrive at the
After clearing a way to the Monastery, and dealing with the greater demon Andariel there, the road is finally free to travel East.
Reaching Lut Gholein by caravan, our hero helps solving a problem with a corrupted 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 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 - Duriel - awaits.
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 . You are faced with the grisly task of collecting body parts from the last uncorrupted priest, in order to create an artefact capable of destroying the Compelling Orb to free the priesthood and reach Mephisto.
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 .
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 Nilathak 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 has been corrupted beyond repair by the demon, and sends the player away to destroy it.