The story of Diablo II takes place soon after the end of the original Diablo. At the end of Diablo, Diablo, Lord of Terror was defeated by a mortal hero. The hero who slew Diablo (i.e. the Warrior character of the first game) drives the soulstone of Diablo (a magical stone containing the soul of a demon or angel) into his own head in an attempt to contain Diablo in his own body. After this event, the hero is rapidly corrupted by Diablo and loses control of Diablo's soul slowly. In the opening cinematic of Diablo II, Marius, the narrator of the story, witnesses the fallen hero (known only as the Dark Wanderer) totally lose control, unleashing the demons of Hell upon a tavern. Marius is the only survivor (it is implied that rather than just being blind luck, the demons were ignoring him), and he feels compelled to follow the Wanderer for reasons he himself does not understand. The new player character is a different hero following in the wake of the destruction, chasing the Dark Wanderer, hoping to put an end to the demon lord within him. The new hero ultimately catches up to the Wanderer outside the city of Kurast but is unable to stop him. The rest of the story is revealed through the four acts, as the player faces not just the demon lord Diablo, but two new major villains, his equally malevolent brothers, fellow Prime Evils Mephisto, Lord of Hatred and Baal, Lord of Destruction. Diablo is determined to free them from their soulstone incarceration, which was forced upon all three long ago, and from which Diablo managed to break free in the first game. The hero travels through different lands to thwart the forces of Hell from conquering the world known as Sanctuary.
The story tells about seven "Great evils" (7 is the number of the powers of hell), some of them are slain in Diablo 2, but two, as of Lord of Destruction, have not yet been seen.
The 7 Great Evils:-
The "Lesser evils"
Duriel, the Lord of Pain (slain in D2, act II)
A very fast and very aggressive enemy. His pure physical strength can easily overwhelm an unprepared player. He attacks using his large claws, and may also freeze the player, making it even easier for him to quickly dispense of opposition. He is found in the true Tomb of Tal Rasha by placing the Horadric Staff into a pedestal.
Andariel, the Maiden of Anguish (also known as Demon Queen, slain in D2, act I)
A fairly powerful enemy using mainly poison based attacks. She uses homing poison arrows, poison spray and the weak claws on her back in hand-to-hand battle. She is found deep in the catacombs of the monastery behind a large set of wooden doors, surrounded by her minions.
Belial, the Lord of Lies (Has not yet appeared.)
Azmodan, the Lord of Sin (Has not yet appeared.)
The "Prime evils"
Mephisto, the Lord of Hatred (eldest of three brothers, slain in D2, act III)
He uses several powerful lightning attacks and doesn't have a melee attack. He can be found in his lair in the temple city of Travincal in Kurast by destroying the Compelling Orb that locked him in his lair. You must use a unique flail called Khalim's Will to destroy it, created after finding the relics of Khalim and transmuting them with Khalim's flail in a Horadric Cube. Beware of the many allies and minions he has throughout the Durance of Hate-the Council Members in particular. Maffer Dragonhand is nearly as dangerous as Mephisto himself.
Diablo, the Lord of Terror (Devil, in Latin, slain in D2, act IV)
The Leader of the Three. He uses the fire element. He is a powerful boss, especially in later difficulties. Beware of his red lightning, which resembles a long red stream of lightning spraying from his mouth, as even characters with good vitality can be killed in one full blast. However this attack is slower than most of his other ones and can be easily avoided. His attacks include: Red Lightning Hose, Fire Nova, Firestorm, Bone Prison, Cold Touch, and a charge-like melee attack. He appears after activating the five hell seals and defeating their protectors, the Lord De Seis (Unique Oblivion Knight), Infector of Souls (Unique Venom Lord) and Grand Vizier of Chaos (Unique Doom Caster.)
Baal, the Lord of Destruction (slain in D2 Expansion: Lord of Destruction, act V)
He is found in the Worldstone Keep. After destroying the five swarms of his minions, he'll walk into the portal to Worldstone Chamber, the battleground. He uses fire strikes and a cold attack, the latter of which causes a severe knock back, as well as freezing. He also summons a copy of himself, capable of using all of his attacks, only adding to the troubles of anyone attempting to slay the Lord of Destruction.