A little while ago, we enabled the Scourge Invasion on the Public Test Realm (PTR).
This event was first released with original WoW patch 1.11, and will happen in WoW Classic concurrent with the release of the Naxxramas raid.
The basic structure of this event is that Necropoleis appear in six different zones, with crystals spawning beneath, and undead spawning around those. Killing the undead allows you to combat the crystals, which in turn allows you to clear the necropolis and defend the zone.
You’ll find scourge invasions marked on the World Map with a purple skull in:
See you there!