The Scourge is one of the two undead factions in Azeroth (the other being the Forsaken). The Scourge was created by Ner'zhul the Lich King and Kel'Thuzad, following the plan of the Lich King's creator, Kil'jaeden the Deceiver, Lord of the Burning Legion. They have since gone rogue, earning the ire of their former masters, and are considered major threats by both the Alliance and the Horde.
Everyone knows about the Scourge. They paved the way for the coming of the Burning Legion. The Lich King, who was an orc warlock named Ner'zhul, sent an undead plague to Lordaeron with the help of a corrupted Alliance archmage, Kel'Thuzad. This attack threw the citizens into chaos: Some contracted a plague that would kill them then raise them as foul zombies (or worse); and others were forced to watch the graveyards erupt, their loved ones returning as monsters. The Lich King then corrupted the beloved prince of Lordaeron, Arthas. In a masterful move, he played Arthas with the help of Kel’Thuzad, convincing him that he served his people by slaying them in cold blood when he realized they had contracted the undead plague. His soul was forfeit when he took up the powerful, corrupted blade Frostmourne with the intent of becoming powerful enough to save his people.
He became a death knight, killed his father in cold blood, and claimed the throne of Lordaeron for the Scourge. When the Lich King’s power began to fail, Arthas investigated and split the Frozen Throne where Ner’zhul was imprisoned. The power of the Lich King entered him, Ner’zhul and Arthas merged, and Arthas claimed the Frozen Throne as the new Lich King. So he’s now the new Lich King and resides in Northrend, the place of his corruption. He, however, is not encased in ice as the former Lich King was, and he commands the Scourge on Northrend as well as the thousands of undead that have overrun the former jewel of the Alliance, Lordaeron.
The current plans of the Scourge are unknown, but all fear the implications of more and more undead, powerful necromancers, and liches populating what used to be their homes. Fighting in war and dying a warrior’s death is one thing. Yet fighting with the knowledge that ones death will not be a clean one, that they will come back as a foul being; that is something else. The Scourge fights everyone, Alliance, Horde, Burning Legion and innocent bystanders. Even with that many enemies, it does not fall. It thrives, because they gain any warriors who die in battle. The Scourge is not invincible, however. They can be cut down, and any dwarf, tauren or gnome would do as much. However they hate the necessity of defiling their own people's corpses to make them unfit for the Scourge to raise again. There are some who cannot bring themselves to do this, for religious or sentimental reasons, and they are foolish; it is better to cut off a friend’s head when he is dead then to cut it off when he is risen again, and can look you in the eye.
Fighting the Scourge is something everyone on Azeroth can get behind. While simply “killing” a creature again doesn’t seem to do the job, as they often have a necromancer on the rear lines to just raise them again, cutting them up can be effective. A couple of good slices with an axe and they’re done for. Fire is also useful, as charred husks do not serve as such a useful army. People do not want to come home to burned fields and shells of houses, but a scorched home is better than no home at all.
The Alliance has a few Knights of the Silver Hand on its side, but this is a group that has experienced more hardships than most during the current dark days. Once the pinnacle of enlightenment, goodness, purity and light, they are forever shamed that the powerful Lich King who sits upon the frozen throne was one of theirs. They wrack their brains on where they went wrong, why they couldn’t see the streak of evil that obviously lurked inside Arthas. If he were truly a paladin, as they are, then there is no way he would have been corrupted. They may have rather a too high opinion of the paladins. Even though paladins are some of the most powerful weapons against the Scourge, with their holy power over undead, the paladins are not what they once were. Some have been driven into obsessive madness, forming the Scarlet Crusade and killing living and undead alike in their eagerness to eradicate the Scourge. Others traveled across the sea with Jaina Proudmoore to help defeat the Burning Legion and now reside on Theramore. They certainly do what they can to destroy undead they discover on Kalimdor, but the Scourge’s numbers there are tiny compared to the lot that are in Lordaeron and Northrend. Paladins are needed to defeat the Scourge, but they are not up to the task right now, and it's unknown how long the Alliance can wait. Reports from Lordaeron say that the Scourge’s goal currently is to hunt down the remaining pockets of resistance, destroy the remote villages, and raise all of the Alliance’s graveyards. Essentially, they are trying to take what was once the most populated continent and turn it into a hell, subverting every available person, dead or alive, to their control. They sow evil, terror and undeath among the living. They dominate Lordaeron and eventually the world. They intend to return the Forsaken to the power of the Scourge. They want to destroy the Alliance, Horde and Burning Legion.
The Scourge was created by Ner'zhul the Lich King. Under the direct control of the Burning Legion, the Scourge's mission was to spread terror and destruction across the world in anticipation of the Legion's inevitable invasion. The Lich King, who ruled the icy realm of Northrend from his frozen throne, created the terrible Plague of Undeath, which he sent southward into human lands. As the plague encroached on the southlands, countless humans fell prey to Ner'zhul's mental control and life-draining sickness every day, swelling the ranks of the already-considerable Scourge.
Though Ner'zhul and his Undead Scourge were bound to the will of the Burning Legion, the Lich King constantly strove to free himself and gain vengeance upon the demons for dismembering his body and damning him so completely. At the Battle of Mount Hyjal, Ner'zhul was robbed of his chance for vengeance when Archimonde was slain and the Legion defeated; but it was at this point that Ner'zhul was finally capable of making his bid for freedom, effectively cutting all ties to the remaining demons as well as the remaining demon lord, Kil'jaeden. Such a move inspired Kil'jaeden to incredible heights of anger, and it was the demon's turn to lust after vengeance. But, thanks to the Legion's defeat, Kil'jaeden was stripped of his capacity to simply bring about swift destruction, and was forced to use more subtle methods to dispose of his rebellious creation.
Shortly after the Battle of Mount Hyjal, Kil'jaeden contacted Illidan Stormrage with an offer too tempting for the Demon Hunter to refuse: Slay the Lich King, and be granted the awesome power of the demon lord. Gathering his formerly-Highborne allies, the Naga, Illidan traveled to the citadel of Dalaran and began weaving a massive spell targeting the Frozen Throne using the Eye of Sargeras. However, the spell was interrupted at the eleventh hour by Illidan's brother Malfurion Stormrage, Maiev Shadowsong and Prince Kael'thas Sunstrider. Though the spell was not completed, enough potent fel energy had been siphoned to cause a fracture in the Lich King's icy prison. This caused Ner'zhul to weaken, at a rapid pace, his power to control his undead. It was during this time that a large band of undead, led by Sylvanas Windrunner, regained their free will and left the Scourge, calling themselves the Forsaken. Desperate, Ner'zhul contacted Arthas, the greatest of his Death Knights, by telepathy and commanded him to return to Icecrown Citadel at once.
However, the matter was complicated by Kil'jaeden once again. Growing impatient with Illidan's failures, the demon lord instructed him, with his allies the Naga and, newly, the Blood Elves, to travel to Northrend and put an end to the Lich King personally. Arthas' undead forces battled Illidan's army at the base of the glacier, the aftermath of which left Illidan wounded and his troops broken. Arthas, victorious, proceeded into the Lich King's throne chamber, where he used the runeblade Frostmourne to shatter the Frozen Throne and free the essence of Ner'zhul. The spirit of the Orcish shaman then bonded with that of Arthas, and Arthas/Ner'zhul became one of the most powerful beings the world had ever known.
Four years after the transformation of the Lich King, Arthas still resides in Northrend, presumably rebuilding Icecrown Citadel. While the status of the Scourge in the northern continent is currently unknown, the undead in Lordaeron — under the command of Arthas' majordomo Kel'Thuzad — continue to hold the Plaguelands from the enemies of his lord Arthas. It is likely that Arthas plans to retake all of the former Lordaeron from the Forsaken, and from there possibly conquer the whole world.
At the top of the organization, of course, is the Lich King, Arthas. He sends orders from on high in his ice fortress at Northrend. His direct underlings are liches, undead mages and necromancers with incredible power to spread plague and command undead armies. It is unknown how many liches there are, however Arthas’s right-hand lich is Kel’Thuzad, who reigns in Lordaeron. He has his hands partially full fighting the Scarlet Crusade — human zealots who would just as soon kill a living creature as an undead one, just to make sure. Kel’Thuzad also battles the Forsaken, who are undead who broke free from the control of the Lich King. Another fearsome undead creature is the banshee. They often lead scouting troops to discover new areas to infiltrate. Other important Scourge members include necromancers, who are responsible for raising the dead and commanding them, and the Cult of the Damned, a bizarre group of mortals, mostly humans, who are insane. They are so fascinated by the undead that they follow them, worship them, even emulate them, hoping to one day be damned as the undead are. They don’t see the foulness and evil. They see power and eternal life.
Most of the undead report to a region’s necromancers or liches, who in turn report to Kel’Thuzad. He reports pertinent information to Arthas, but he doesn't inundate his lord with all of the details of the happenings on the continents. If they take over another city the size of Stratholme, or re-enslave the Forsaken, that would be news worthy of telling the Lich King. The rogue undead, the Forsaken, are wild cards. Arthas lost some power over the undead before becoming the Lich King, which allowed one of the more powerful banshees, Sylvanas Windrunner, to escape his hold. She freed many more undead creatures, and currently leads the Forsaken from her tunnel complex underneath the former court of Lordaeron. While they are not part of the Scourge and do not fall into the Scourge organization, they work along the same lines — kill the living and dominate the whole of Azeroth. Arthas sees the Forsaken as something akin to lost sheep, and wishes to bring them back into the Scourge. He has many plans for Azeroth, so he can’t focus entirely on Sylvanas, so its unknown when he’ll turn his full gaze to her and her followers. The interesting thing is that the farther from a lich or a necromancer the undead get, the more disoriented they are. They lose the command of the Lich King, but they don’t necessarily become free. Once the will of the necromancer or other controlling force leaves their head, they have no memory of their former selves, and they shuffle around, mindless husks, searching for someone to lead them. Only necromancers can command the undead.
Arthas leads the Scourge in Northrend from the Frozen Throne, those beasts are everywhere in Icecrown. The Scourge’s secondary base is the continent of Lordaeron, in the city of Stratholme, Arthas’s first conquest. It was a bustling city of 25,000 people, and now it’s gone. There are few if any remaining mortals there, except for those of the Cult of the Damned and mortal necromancers. Lordaeron’s north and eastern areas are practically abysmal with the Scourge. Their foul stench permeates the air. Yet the Scourge has reached all areas of the world, dripping down through Khaz Modan and into Azeroth, and even west to Kalimdor. Arthas wants his finger on everything happening in the world, and he has the forces to do it. About the only place the Scourge doesn’t go is the Undercity in Lordaeron, but there are so many Forsaken there, that one couldn’t tell if there is Scourge presence there or not. Though, the Forsaken might know.
The members of the Scourge are easy to spot, smell and identify. And it doesn’t take much to join. All it takes is to either love the Scourge or be undead and under the Lich King’s spell, and you’re in. There is not a lot one needs to do to be in this organization. Some of the first members were mortals who had fallen in the undead plague. People sickened from the plague and died, only to rise as zombies. This city-razing technique proved to be not as efficient as the Lich King desired, apparently, because he started telling his necromancers to desecrate graveyards and raise up even more members. Only two things seem to separate a Scourge servitor from the Lich King’s will: whatever faltering in the Lich King’s power allowed the Forsaken to free themselves from his grasp, and destruction of their bodies. Scourge creatures are bidden to do the Lich King’s will, and they do this with a blind fervor. The Cult of the Damned, is an enigma. No one knows why they do what they do. Perhaps their mothers didn’t love them enough? They didn’t get the right apprenticeships? The love of their lives spurned them (or got turned into undead)? Whatever the reason, they willfully follow the Scourge, dressing in dark robes, doing their bidding, helping to spread the plague into more and more cities.
The Scourge is primarily concentrated in the Plaguelands. Their primary fortification and "capital" is the ruined city of Stratholme, currently split between Baron Rivendare's forces and the humans of the Scarlet Crusade. The Scourge has also been seen in Tirisfal Glades and Silverpine Forest in Lordaeron, and all along the Dead Scar in Quel'Thalas, which runs from the Ghostlands all the way into Eversong Woods to Silvermoon itself. They also have been reported to have a presence as far away as the Barrens in Kalimdor; "ambassadors" from the Scourge, as well as powerful undead beings, have fortified themselves (along with the plagued Razorfen quilboars) in Razorfen Downs.
The necromancer-lich Kel'Thuzad, the majordomo of the Lich King, controls the Scourge in Lordaeron from his citadel of Naxxramas, floating over Stratholme and the Plaguewood in Eastern Plaguelands; the high elf traitor Dar'Khan Drathir, slain during the Third War, rules the Scourge in Quel'Thalas from the fortress of Deatholme in the southern Ghostlands, to the northeast of Naxxramas; Amnennar the Coldbringer, a lich like Kel'Thuzad, rules Razorfen Downs.