The Plague of Undeath was a plague created by Ner'zhul and the Burning Legion that was used, mostly, in Northern Lordaeron leading to the fall of the Kingdom's of Lordaeron and Quel'thalas. The release of this plague resulted in the creation of the Undead Scourge as inhabitants of affected lands were killed and turned into terrifying creatures of undeath.
Origins[]
Unlike the Ancient Plague that hit the continent in the past, the Plague of Undeath was a plague on a level unprecedented in Lordaeronian history. Not only did it destroy the population of Lordaeron, still in a recovery from the 2nd War, by around ninety-five percent (as some scholars estimate) it also saw those millions who died, men, women and children, being raised into the ranks of the Undead Scourge. The plague that struck Lordaeron is a topic of incredible debate amongst scholars across Azeroth.
While it has been confirmed to be both magically and organically based, the Plague of Undeath has roots in the Nathrezim’s dabbling with necromantic magic as early as at least the War of the Ancients. However, it is seen as being created by Ner’zhul and first saw use in Northrend before coming to Lordaeron by way of the Cult of the Damned. There is still debate, of course, whether that Ner’zhul (the Lich King) was simply given the finished work the Nathrezim had been preparing for thousands of years and was the first servant of the Burning Legion to release it en masse or if it was created by him personally.
While necromancy has been known to humanity since around the time they were taught magic by the Elves of Quel'thalas, and we see examples of this with those like Hector the Wretched, the Plague of Undeath is something far more powerful than anything seen by humanity. From the study of Argent Plague Doctors and Cenarion Plague Shifters, it is seen that fungal spores, originating from what is now the Fungal Vale, were manipulated by the Cult of the Damned, to help spread the plague, resulting in massive mushroom formations that are seen across Lordaeron’s Eastern Plaguelands.
These fungal spores have allowed the Plague to take hold faster and spread blight, destroying the area to the point where those traveling in the Plaguelands, in the past, needed to wear protective masks to stop themselves from inhaling the spores. By the time of the invasion of Pandaria by the Alliance and Horde, Cenarion and Argent work has progressed to the point where the spores, while dangerous, no longer fill every point of the Eastern and Western Plaguelands. Massive concentrations of spores still exist within the Fungal Vale, Noxious Glade and Plaguewood and the Argent Crusade has issued countless warnings and advice to those looking to travel into these areas. Throughout the rest of the Plaguelands, the spore count has fallen considerably and the air is more breathable, without protection, than it was in the past. Unfortunately, with strong northern winds, the Crusade has created a weather service to give warnings when spore concentrations would be swept across the Plaguelands.
Before plagued spores were released, the main way to transport and spread the Plague was from the use of grain. Cultists infiltrated and took over grain distribution in places like Andorhal where the thousands of bushels brought in would be dipped into secured vats of plague and then packaged. For months cultists gathered this plagued grain in secret and waited to distribute it. Upon a given signal, these cultists released thousands of bushels of plagued grain and sent it out across Lordaeron. It was by this deception and patience that allowed the Plague of Undeath to quickly tear apart Lordaeron in a short span of time.
This patience on the part of the Cult of the Damned ensured that those in the furthest reaches of Lordaeron would receive plagued grain the same time as the settlements around Andorhal. Of course in some areas the grain did not go through distribution centers like Andorhal, it was at these locations where cultists practiced the same trickery at Andorhal, releasing plagued grain as soon as the signal was sent out to begin the Scourging of Lordaeron.
Reference[]
This expansion of what we know about the Plague of Undeath is mostly all fanon and is the continuation of the Lordaeron Fanon Expansion, you are not forced to listen to any of this if you choose!