You hve been playing WoW Classic for a while from james's blog

You do this by killing Scourge in the area until four Cultist Engineers spawns that you will have to eliminate one by one. The mobs won’t be hostile at first and you’ll need to WOW Classic Gold talk to them and offer them Necrotic Runes in order to have them transform into elite mobs known as Shadows of Doom. Kill all of the elites to temporarily clear the area.

Shadow of Doom mobs drop gear that increases your damage against Undead. Make sure to get as much of that as possible because you’re going to need it in Naxxramas. In addition, the mobs also drop Corruptor’s Scourgestones. You’ve probably seen these before if you’ve been playing WoW Classic for a while. If not, just hang on to them because you’ll use these to increase your rep with the Argent Dawn.

If you’re looking for even more gear, roam around the invaded areas and keep a look out for the new Rare mobs as these also drop items that increase your damage against the Undead. There are four items that can drop from these mobs and, unlike the others, these are BoE. Given how useful they are, these items will probably be very expensive, so it’s probably a good idea to stock up on some WoW Classic gold beforehand if you plan to Cheap WOW Classic Gold buy them from the auction house.