A Story Summary ( the official story is below )

After the attempt by the hero in the first game to entrap Diablo within the body, the hero realized that Diablo was simply too powerful. Seeking help, the hero traveled East, when finally Diablo escaped and took control of the heroes body. In Diablo 2, the story starts out somewhere in the East, where Diablo is building a new and all powerful army. However, this time there are now 5 new character classes, all of which is unique, each of which contains different strengths and weaknesses. The story in Diablo 2 goes through four acts, with each act comprising of a town and its respectful dungeons. Also each act will also give a little more information on Diablo's history and sinister plans, which include freeing his evil brothers Baal (the Lord of Destruction), and Mephisto (the Lord of Hatred). Together, they form the evil triumvirate known as the Prime Evils. As the game's hero you will have to track Diablo through each act and town and attempt to defeat the Prime Evils, and their armies. Your hero comes into the game as a powerful and experienced adventurer who has heard of the Lord of Terror's renewed rampage, and has set out to stop him. The new character classes in which enter the game are the Amazon warrior, the Barbarian, the Paladin, the Sorceress, and the Necromancer. Your first task begins in the mountains at the Sisters of the Sightless Eye monastery, where the Lesser Evil Andarial (an evil rogue) has seized control, and is using the rogues for the purpose of Diablo. Andarial soon corrupts all the captured rouges, but a small band of survivors forms around the monastery, creating a makeshift camp and Diablo 2's first town. And so it begins... You need to uncover Andariel's hand in the fall of the monastery and free the Sisters. Once you defeat Andarial in the town's final level, you'll move on to subsequent acts and locations in the East. All the characters will have their own set of fighting abilities or spells.

Diablo 2 will have four acts, which take place in four different and unique locations. The locations are presented in a linear progression across the distant lands to the East, meaning that before the hero can enter another town, the act from the previous town must first be completed. However, after the Act is completed, the hero may return to any previous town. Rather than having a single town the hero may return to, there is a different one in each Act (for a total of four), each with completely unique characteristics, environments, and each populated by new townfolks . Also, although no NPC's (non-playing characters) from Diablo are planned to return, Cain the Elder and Wirt the pegged legged boy are supposedly scheduled for a return.

It has begun...

The Official Story

"The three prime evils of hell once roamed the ground we stand
upon. Also known as ,"the Three," these evils would have taken over
the world with their army of the undead fighting by their side. Had it
not been for the horodrim, who skillfully trapped the three into
artifacts known as soulstones, the world would not be like it is
today: for the three spread nothing but sickness and poverty
everywhere they went.

Soulstones are artifacts used to trap the soul of a being, forever (as
long as the soulstone remains in one piece). The horodrim found the
three to be much to difficult to kill in battle, so they trapped them
inside the souldstone instead. They then hid the three soulstones at
different corners of the world.

The strongest of the three, Diablo, was not easily contained. His
presence was trapped in the soulstones, but his essence was to
strong to be held by its magical walls. Even as he sat trapped in the
ruby stone, they could hear his voice in their heads. The horodrim
knew that he could not be contained forever, and decided the best
way to take care of this monster was to keep him in their presence,
as they were the only ones strong enough to resist then temptations.
Diablo sat trapped in the soulstones underneath the horodrim
church, and life continued that way for some time: things were
peaceful. But time passed on, and so did the horodrim. Diablo felt
their presence slowly die off, and as he was no longer contained by
their strength, he began to search the land for a weak mind to help
him escape. After a quick search in the above town of Tristam,
Diablo settled on the lord of the land, King Leoric. Mortal minds are
no match for the prime evils, and it didn't take long to find a weak
minded ruler to follow Diablo's commands. One night, Leoric was
seen running into the church, and down into the dungeons. He was
never seen again.

All at once, an evil glow of death surrounded the church at the
mighty town of Tristam. And soon, the town was attacked by an army
of evil demons: who all but killed everybody except a lucky few.

Far out in the east,the sisters of the sacred eye heard the news. These
rouges ventured out to Tristam, and into the dungeon to slay
Diablo, and the evils underneath the church. Many died in the
attempt: but a lone heroic rouge destroyed Diablo in the final battle.

There was however, a problem....

The rouge found the soulstone smashed into the head of leorics sun,
she did not know how to destroy Diablo because she was not
powerful enough to. She then realized there was only one way to
contain the beast: and with one forceful thrust, she jammed the
soulstone into her head, and prayed that she could withstand his evil presence.

All at once the essence of Diablo filled her thoughts, and pestered
her with every step. She could here his voice: feel his blood flow,
hear his thoughts. She realized she could not stand this forever, and
quickly set forth eastward to seek help.

She overestimated herself, and Diablo eventually over-powered her,
broke free and took over her body. Now, more powerful than ever,
Diablo went mad with rage. He swore he would bring the prime evils
back to the mortal world of men, and they would conquer the world
in full glory. He is out there right now, rounding up an army of pure
evil. It is up to us to stop him.

And so begins the story of Diablo II..."