This is really just an excuse for a bunch of screen shots, and a little exposition for my friends who read this blog but don't play WoW.
The Scourge is an eclectic army of the undead, created by some members of the demonic Burning Legion as their next weapon in the war against Azeroth and its inhabitants. Many of the Scourge are mindless, but not all—it's got officers with substantial powers of personal choice, and even in the ranks there are individuals who can think for themselves at least some of the time. The supreme commander of the Scourge is the Lich King, an entity fusing the corrupted souls of human paladin Arthas and orc shaman Ner'zhul. (You can see the state of things with him in the intro video for Wrath of the Lich King. The voice is that of King Terenas, Arthas' father.)
The Scourge is strongest in Azeroth's northern reaches. The Lich King and his Frozen Throne wait way at the top end of Northrend, the continent being opened up in the new expansion pack. In the currently accessible realms, the northern zones of the Eastern Kingdoms are most likely to be Scourge-Ridden. The Plaguelands are what used to be the four northern human kingdoms, including Arthas' own Lordaeron. But every so often the Scourge reaches out in an effort to grab more territory. Its mobile necropolises go ranging far and wide, unleashing waves of summoned undead who must be driven back before the crystals that do the summoning can be destroyed. This is the second such outreach since the end of the Third War (which was Horde + Alliance versus the Burning Legion). It probably won't work in the sense of giving the Scourge any permanent gains, but the effort ties up a lot of resources anyway.
This is the necropolis parked outside Thunder Bluff, the tauren capital:
This is the one outside Orgrimmar, capital of the Horde, taken from wyvern-back as Tivara was flying across Durotar:
And this is one in the Tanaris Desert, in mid-attack:
Here's the necrotic shard at the center of one beachhead in Tanaris. Notice that there are lots of corpses around it and nobody standing up except Tivara and Sagarmatha. Beat-down of the shard's last defenders is about to begin:
This is another necrotic shard, this one in the Blasted Lands, with the clearing still in progress:
Each necrotic shard has health, just like characters and monsters, linked to the undead it summons. Each time one of its summonees gets killed, it takes some damage. So the first step is to kill a lot of them—two hundred, give or take:
The mound of re-dead bodies beneath our banner shows this work in progress. The banner itself, by the way, comes from the Argent Dawn, a cross-faction union of dedicated Scourge foes, who are among the few genuine heroes in the game. It's always a pleasure to work with them. People who collect enough of the necrotic runes that empower these invaders can use them as currency to buy useful tools from the Argent Dawn, including battle banners. When they're planted, they unleash a consecrating fire just like the one paladins can invoke. Very handy.
When the necrotic shard is broken enough, Scourge engineers teleport in to try to re-energize it. Notice that the shard is now purple-black:
The engineers keep working regardless of what's going around them...
...until someone burns a few necrotic runes to break their concentration. At this point the engineer releases the Shadow of Doom lurking inside:
When all four are defeated (if, of course, they are), the necrotic shard collapses, and nothing remains but the great smell of Brut:
When enough of its necrotic shards have been destroyed, an invading necropolis' commander packs it in for the day, and that zone is safe for now. For a few hours, at least, until the necropolis is back with a fresh set of shards.