The Scourge Invasions of Azeroth were the conquests led by Arthas Menethil against the Grand Alliance and New Horde that sparked the War against the Lich King.
The first Scourge Invasion heralded the opening of Naxxramas, the citadel of the dreaded lich Kel'Thuzad. The Undead Scourge poured forth from the citadel and nearly overwhelmed the forces of the Argent Dawn at Light's Hope Chapel.
During the invasion, several regions of Azeroth came under attack by Scourge forces. Members of the Argent Dawn organized a worldwide counter to the Scourge invasion, keeping an eye out for any necropolis sightings and passing on their information to all adventurers willing to aid them in their struggle.
With each victory against the Scourge, the defense grew stronger. As more and more invasion attempts were beaten back by the defenders, the Argent Dawn was be able to bestow increasingly more powerful blessings upon those fighting the invaders. The mortal races focused on clearing the Scourge camps all over the world that sprang up beneath each necropolis, eventually the invasion was effectively repelled.
Necropolis invasions, Necrotic Runes, and the Shards[]
During the Scourge Invasion, numerous lands fell under attack by one or two flying necropolises each. The zones that were attacked were the Eastern Plaguelands, Burning Steppes, Blasted Lands, Tanaris, Azshara, and Winterspring.
Agents of the Argent Dawn were sent to all the major cities and assisted both Horde and Alliance where the necropolises were attacking. Additional agents were dispatched to the various zones that fell under threat to this invasion. There were also necropolis over the capitals. The ones over the major cities seemed to be permanent for a time, and they constantly spawned weak scourge to assault the cities and surrounding towns.
Assaulting the Necropolises[]
The necropolis were accompanied by Necrotic Shards, which were hidden on the ground near each of the flying citadels. These shards were always protected by a large number of undead.
The Shards near the undead were weakened as the undead were killed. When the strength of the shard reached zero, four Cultist Engineers typically appeared and started to heal it. Protected by necrotic runes, each of the runes needed to be disrupted before the engineers could be dispatched. When the engineers fell the Scourge called upon their second wind, operating with a hive mentality they summoned Shadows of Doom to protect the powerful Necrotic Shard helping to keep the Necropolises afloat.
Destroying all four Shadows of Doom will destroys a Necrotic Shard, the act of destroying a Shard dramatically prevented new Scourge from spawning there (at least for a short time).
Destroying all the Necrotic Shards attached to a necropolis will destroy the necropolis. With assistance from the Horde and Alliance the Argent Dawn destroyed many necropolises. The Scourge eventually deployed more necropolises however, so the cleansing of the Scourge Invasion was a lengthy process.
The Second Scourge Invasion[]
The capitals of Azeroth began receiving mysterious packages with infected grain. As their citizens ate grain from the infected packages, they were turned into bloodthirsty ghouls if not treated by members of the Argent Dawn. With all the major capitals thrown into chaos, necropolises appeared around the world, triggering a second Scourge war. Adventurers from both the Alliance and the Horde fought back the disease and the advancing Scourge armies. Outraged by this treachery, Warchief Thrall and King Varian Wrynn planned an invasion on Northrend to deal with the Lich King once and for all.
The final straw occurred when the Lich King directly attacked the capitals of the Horde and the Alliance. Orgrimmar and Stormwind were attacked by a large Scourge army consisting of frost wyrms and abominations. The Battles for Stormwind and Orgrimmar were won by the brave adventurers and their leaders once more, thus forming the Horde Expedition and the Alliance Vanguard.
Scourge invade Orgrimmar