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.