With the release of Naxx and phase 6 coming to official servers sooner rather than later, I'd like to do a quick refresher on the scourge invasion event which are going to be happening in the lead-up to the phase 6 release. From what I've seen they've already been testing them on the ptr servers so I'm sure they're going to be rolling them out on the open servers pretty soon.
Now to be prepared for the event here is what's going to happen. 6 necropolises are going to spawn in 6 regions, namely the Eastern Plagueland, Burning steppes, Blasted lands, Tanaris, Winterspring and Azshara. In these regions you're going to find a necropolis floating in the sky with a crystal underneath, that's going to spawn undead enemies until defeated. All of these crystals are also going to be marked with a purple skull on the map.
There are probably going to be a lot of players interested in destroying these crystals so you'll probably have to crowd if you're going to want to get at them. If you'd like a more serene experience of dealing with them, i'd recommend hopping on a private server and talking to the gamemasters there about triggering the event. I'd recommend this wow server list for finding it since it's where i found the server i play on quite regularly.