Through the boards can be seen a few questions concerning the Scourge Invasion. Why is Arthas and his Scourge invading Azshara, Tanaris, Eastern Plaguelands, Burning Steppes and Blasted Lands?
To be honest, I haven’t seen any quests that substantiate why these locations are getting invaded, which is a shame. At the moment they are just there so you can farm necrotic crystals to buy special trinkets and gear from the Argent Dawn Quartermaster. Nevertheless, I do have an explanation from a lore standpoint.
Azshara: Back when the Gates of Ahn’Qiraj patch was released, or approximately around that time, Azuregos the blue dragon was added as an outdoor raid boss. From there came the lore that Azuregos was protecting various artifacts from the golden age of the Highborne empire, and that one of the vials of eternity was missing in this area. Arthas the Lich King would go at any lengths to acquire this Vial of the Well of Eternity if the legends are true. Three vials were used by Illidan at the top of Mount Hyjal. One vial was used by Dath’Remar Sunstrider to create the Sunwell in Quel’Thalas. One vial was given by Illidan to Lady Vashj and one vial to Kael’Thas Sunstrider. If my math isn’t failing as usual that is six vials. One is missing and unaccounted for, which strengthens the legend that a missing vial might be somewhere in the Kalimdor region: Azshara. The Warcraft: The Sunwell Trilogy (Tokyopop manga) reveals that King Arthas the Death Knight was able to infiltrate Quel’Thalas with the help of a high elven traitor: Dar’Khan Drathir. Dar’Khan was sent (ahead of Arthas’ arrival to the Sunwell) to syphon the energies of the Sunwell. Arthas the Lich King has always wanted to get ahold of the magic of the Sunwell. Getting a Vial of Eternity would be even better.
Before the Great Sundering, the night elf city of Eldarath flourished in the land that is now known as Azshara. It is believed that many ancient and powerful Highborne artifacts may be found among the ruins of the once-mighty stronghold. For countless generations, the Blue Dragon Flight has safeguarded powerful artifacts and magical lore, ensuring that they do not fall into mortal hands. The presence of Azuregos, the blue dragon, seems to suggest that items of extreme significance, perhaps the fabled Vials of Eternity themselves, may be found in the wilderness of Azshara. Whatever Azuregos seeks, one thing is certain: he will fight to the death to defend Azshara’s magical treasures.—source
Tanaris: If you have done the quests in Andorhal, you met Chromie and completed A Matter of Time. It is revealed that the Scourge attempted to alter time in Andorhal. Tanaris is a very strategic location for a Scourge invasion … Caverns of Time. Imagine if Arthas the Lich King could hijack it to alter the past, and conquer other timelines and realities? (More after the break)