It would be bad enough having one kind of undead, but the world of Azeroth is unlucky enough to suffer from many varieties of the walking dead. The Scourge (formerly lead by Arthas Menethil, now chained by the will of Bolvar Fordragon as the Lich King), the Forsaken (servants of Sylvanas Windrunner who began their unlives as Scourge), and the Risen (former Scarlet Crusaders raised into undeath by the demon Balnazzar) are the most numerous subgroups, but they aren't the only examples of their kind.
Undeath in Azeroth comes from necromantic magics, which have long been known to the demons of the Burning Legion. Indeed, the Scourge and the Risen were both born directly or indirectly from Burning Legion-affiliated demons, while the Forsaken derive from the Scourge and thus owe their unlives to demonic forces as well.
There have been undead on Azeroth since the War of the Ancients. Many are bodiless spirits; still others are risen flesh that refuses to accept its own demise. Not only does Azeroth have these contending groups of undead to deal with, but within each are many variations on the theme, from the ghostly val'kyr and banshees to the vampiric san'layn to the monstrous abominations. The only thing that can be said to be shared by all undead is their state, stranded between life and death.
To make it clear: The Scourge are hardly the first undead to blight Azeroth. Captain Varo'then, the commander of Queen Azshara's personal guard, was so loyal to the Queen that he was willing to go so far as to destroy Kalimdor itself in her name, and the land itself cursed him to never rest within it, rendering him a ghost that endured for 10,000 years.
The state of undeath
The state of undeath isn't coupled to any of the many methods by which it has been reached. Some undead are self-created, such as the lich Naberius. Naberius is notable for having succeeded where even Kel'Thuzad failed in reaching undeath purely through his own actions. Even Ner'zhul himself only became undead after being tortured and torn asunder by Kil'jaeden, who then created him as a unique undead being in the form of the original Lich King, but Naberius brought himself to undead and the form of a lich.
Similarly, Meryl Felstorm (once Meryl Winterstorm) was a human mage who lived during the Troll Wars who directly kept himself in a state of undeath through his own magics. This is the rarest way for one to become an undead. Usually the state is bestowed or forced upon one by a powerful wielder of necromantic magic such as Kil'jaeden, the Lich King, or one of the Nathrezim.
One of the distinctions between groups of undead is their level of autonomy. The Risen, who in life were the Scarlet Crusaders deceived and used by Balnazzar in his cover identity of Saidan Dathrohan, display almost no free will and very little active intelligence. Even the Scourge under the various entities calling themselves the Lich King had members who displayed intelligent service to their master (such as Kel'thuzad as a Lich, Lana'thel, or Anub'arak), but the Risen lack even this level of independence, no doubt because of the experience Balnazzar had losing control of the Plaguelands to Sylvanas Windrunner and her Forsaken.
While the Forsaken seem fanatically loyal to Sylvanas, with a very few exceptions, they definitely appear to retain the most independence of thought and action of any of the undead born from Burning Legion necromancy. Essentially, the Forsaken were created in the same way as the majority of the Scourge, infection via the Plague of Undeath originally distributed by the cauldrons used by Kel'thuzad and the Cult of the Damned.
Loyalties and alliances
However, the modern Forsaken now have an entirely new method of creating more undead, as the Banshee Queen has made an alliance with a group of val'kyr formerly in service to Arthas. These val'kyr can directly raise the dead as free-willed Forsaken, but while it seems that the ones raised in the Deathknell area can choose whether to serve Sylvanas or not, the ones we see raised on Fenris Isle seem to immediately convert to fanatical worship of Sylvanas.
With the Scourge in disarray following Arthas' death and Bolvar's seizure of the Helm of Domination, the Forsaken are the largest active contingent of undead. As members of the Horde, the Forsaken have expanded onto the shores of Northrend as well as throughout the northern regions of the Eastern Kingdoms, holding much of the former Lordaeron, Gilneas, and the Alterac region and making inroads into the Arathi Highlands.
Another group to be discussed are the death knights of the Order of the Ebon Blade. These death knights, similar to the Forsaken, have regained a semblance of their former independence and have banded together. While death knights serve as members of both the Horde and the Alliance, all of the death knights that regained their wills after the Battle at Light's Hope Chapel are considered members of the Ebon Blade.
Originally dedicating themselves to vengeance against the Lich King for the undeath he forced upon them and the atrocities he used them to commit, the Ebon Blade is now in a position of uncertainty. With Arthas dead and the Scourge in shambles, they would seem to have no real purpose for existence. At present, Darion Mograine has not made any moves toward establishing a new purpose for the Ebon Blade in a post-Cataclysm world.
Next week: the nuts and bolts of the undead.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.