The only game ever made that is perfect!
This is exactly what hapens in LoD. After beating Mephisto and Diablo in the original, the third of the prime eavils appears. This is a bad thing for the people in that world, but an oh so great thing for us gamers. The expansion allows you to continue your journey from Mount Harrogath, through the barbarian wastelands, the icy cripts and demon caves to the worldstone. this worldstone is the thing Baal is after, since it can open the gates to hell(that seems worth stopping him no?).
The expansion introduces two new characters. The assasin and the Druid.
The assasin can use Marshal arts with her special claw weapons and can set traps. The Druid has powers of the nature and has shapeshifting skills. Powers of the nature include control of nature(which results in summoning skills) and elemental skills( Volcano's , Tornado's and the powerfull armageddon.) Shapeshifting skills allow him to turn into a werewolf or werebear. These changes increase his damage, attack speed and life.
Off course there is more to an expansion than new characters. There are also alot of new items available, going from new weapons, class specific items, charms to runes that can be socketed into items or can be used in cube recipes.
For those who play online, this also means you now have the possibility to fight über Diablo and take the tristram challenges which puts you face to face with all three of the prime eavils at once(off course they are much stronger than before)
I can keep on talking about this game for hours, but you don't have time to read that long, do you? So, all you really need to know is that this is a great game, with massive replayability and off course, alot of fun.