The Scourge was one of the most iconic collective Warcraft villains for many years. Introduced in Warcraft III, the army of the undead provided a very different foe from Fel-crazed Orcs (and came about just when the Orcs were ditching their fel-craze in favor of more traditional shamanism and proud warrior culture.)
(There will be spoilers here for some Mage and Warlock artifact quests as well as a lot of stuff in the Death Knight class campaign.)
Of course the story of the Scourge was embodied within that of Prince Arthas, who is introduced as an idealistic young paladin. As crown prince of Lordaeron, he gets to be trained by Uther the Lightbringer himself. But while his ultimate turn to evil comes through a fantasy-story contrivance (namely, Frostmourne robs him of his soul,) in truth he had been heading down a dark path of obsession since before his journey to Northrend began.
While he literally loses his soul upon taking up Frostmourne, what seals him as the monster he has become is the murder of his own father.
In fact, Arthas winds up making Lordaeron City into his capital, given that he is, technically, now king of Lordaeron. I actually would love to see what that was like - while Arthas found himself on the run by the start of the Undead campaign following that moment, it would be fascinating to see what happened as the capital was swiftly turned by his forces while the unprepared armies of Lordaeron tried to make sense of and deal with this implosion of their kingdom. After the Third War, the sewers of the capital have been excavated and turned into what we now know as the Undercity, and it's not until Illidan's assault on the Frozen Throne that Sylvanas and the Forsaken recover their free will and rebel, forcing Arthas to retreat and come to the Lich King's aid - an act that ends with his ascent as the new Lich King.
Wrath of the Lich King ended with Arthas dead, but they very pointedly left the Scourge intact. Bolvar Fordragon - long thought dead - instead takes on the crown to become the third Lich King, calling himself the Jailor of the Damned.
While players could easily watch the cutscene at the end of that boss fight (Wrath was the first expansion to have actual cutscenes, and the post-Lich King one established the tradition of having a cutscene for the final boss of each expansion. As a side note, a cutscene that will play at the end of the Gul'dan fight in Nighthold has leaked, though I'm going to save my discussion of it until we get closer to that raid's release.) Canonically, though, only the people up at the top of the Frozen Throne - players who beat Arthas, Tirion, Bolvar himself, and later Sylvanas Windrunner - actually know for a fact that there is a new Lich King. As far as the average commoner knows, the Lich King and the Scourge are basically no longer around, or at least the remnants are just waiting to be eliminated.
For three expansions, we didn't hear anything out of Bolvar. The Scourge does still exist in Eastern Plaguelands, but it is a tiny remnant with individual undead creatures vying for control of what remains. The Cult of the Damned is still around, what with the reintroduction of Darkmaster Gandling in Mists of Pandaria. But the Scourge sure doesn't look like world-threatening juggernaut it once was. Bolvar seemed to be doing his job right.
In fact, Warlocks who go after the Scepter of Sargeras (Destruction Artifact weapon) actually go and fight some Cult of the Damned members around Scholomance, discovering that Gul'dan is trying to recruit them for his new Shadow Council.
Mages who go after Felo'melorn (Fire Artifact weapon) actually track down a distant cousin of Kael'thas who went off to pursue her family's weapon.
Death Knights, however, get the fullest view of the current state of the Scourge. Because while the Knights of the Ebon Blade were instrumental in fighting the Scourge in Northrend, with this new leader, they have an ally. In fact, it is the Lich King who declares you Deathlord, charging you with acting as his champion in the fight against the Burning Legion. The Knights of the Ebon Blade hold a healthy level of skepticism toward the Lich King, but that does not stop them from accepting his cooperation.
Like Arthas, Bolvar was a Paladin before he became Lich King. But Arthas had an intermediate step of becoming a Death Knight. Arthas had dedicated himself to evil, and took the crown as a reward. Bolvar took the crown as a sacrifice to keep the world safe and keep the Scourge under control. But while he has clearly kept the Scourge away from the vulnerable mortal populations of the world, and his intentions have been the best, Bolvar is by no means the same nice guy we had known before.
Fire mages are truly outsiders when they travel to Icecrown Citadel to retrieve Felo'melorn, and Bolvar warns them that he will only tolerate them briefly within his domain. Should they fail, he says, they will be raised and join the Scourge, just as he had with Lyandra Sunstrider.
While all Death Knights interact with the Lich King, it is the Frost quest that has you journey through Icecrown Citadel. The Frost artifact swords are forged from the shards of Frostmourne, and while the vast majority of souls within the blade were freed when Tirion Fordring destroyed it with the Ashbringer, some still remain. With the Citadel, Bolvar has no pity for these lost souls, commanding the Death Knight to put them down, forcing them to submit and renounce the light.
In a sense this is just practical - if these souls are tainted and thus can only find any sort of redemption in service to the new, less world-endy Scourge, you're essentially showing them tough love. But it's really, really tough.
And we should remember that morality is a little different for the undead. A Death Knight player character might be a hero in a real sense - rescuing children and fighting off evil - but one gets the impression that there's not really any great sense of benevolence there. Death Knights fight evil because it is a purpose that has been left to them, and that seems to be what motivates them to keep on going.
It's actually pretty interesting - one of the big Death Knight class quests has you travel into the Scarlet Monastery and raise the corpse of Sally Whitemane to serve as one of the Four Horsemen. Despite the fact that you guys are the undead and they're the really religious ones, it's Darion Mograine who talks to the newly-undead Whitemane about how she has just been offered a chance at redemption - one that she accepts pretty readily (the Four Horsemen each become class champions.)
But the Scarlet Crusade has always been WoW's default "yes they're living humans but it's still ok to fight them" villains. Raising the High Inquisitor to serve as a Death Knight is the kind of ironic punishment (or penitence, depending on your interpretation) that you can sort of get behind.
What's far more compromising is the final mission for the Death Knight campaign. Most of these campaigns have you fighting some demonic force. In this case, you actually raid the Paladin class hall. You slay several perfectly innocent - allied, in fact - paladins in an attempt to retrieve the corpse of Tirion Fordring to serve as the leader of the Four Horsemen. Ultimately, this goes about as well as the last time the Undead attacked Light's Hope Chapel, and Darion Mograine is killed by Lady Liadrin, giving you not only a pretty useful corpse to raise, but also a Mograine whose father was the last leader of the Four Horsemen.
Having done 9/12 of the class campaigns, this is definitely the only time I've seen one class order literally attack another. Ultimately, the attack is a failure - which is probably the only way the Silver Hand and the Ebon Blade are ever going to be able to put this behind them - but it does show you that being a Death Knight really robs you of the values that you may have once possessed.
So what does that mean for the Scourge? For one thing, does the fact that the Ebon Blade is taking orders from the Lich King mean that they're actually part of the Scourge again? And what exactly does the Scourge represent as a whole? The Ebon Blade has been basically an anti-hero force within Azeroth since what I guess we're now going to have to call the First Battle at Light's Hope. They've been a net-positive for the world, but not without their problems. Will the Scourge become that as well?
In some future expansion, am I going to grind reputation with the Scourge and get a tabard when I hit exalted?
Or was Bolvar's ascension just a stopgap? Is the Scourge as a whole going to devolve back into the world-threatening army it once was? And given that Death Knights clearly will still be played by players, that would mean that either the Ebon Blade would have to abandon the Scourge, or players would have to abandon the Ebon Blade.
And this is all just the Scourge proper. We're not even talking about its most successful off-shoot, the Forsaken, whose leader recently became Warchief of the Horde but still seems to be concerned mostly with her own future.