The Scourge Invasion was part of the original Naxxaramas patch release event which began on June 20, 2006. While world events have been happening to help announce big patches or expansions since alpha, this particular event is the only one which has been repeated.
The general gist of the event was that the scourge were invading and needed to be pushed back, the ultimate part of which was to of course storm Naxxaramas itself. There were various stages which featured different quests and mobs, useful rewards, extra bosses in some dungeons, and an opportunity to farm Argent Dawn reputation.
However, as I had literally just begun playing the game the day after the event started, I had no idea what was going on, but it definitely caught my attention when I made it to Stormwind for the first time and saw the necropolis floating outside the city with purple runes below.
While the necropolis, runes, and ghosts were interesting, what was shocking was that the undead had made their way into the city and were engaged in fighting everywhere–from the Park to the Keep! It was one of the events which made the world real to me; even the capital city was not fully safe if this was happening! I wanted to help the NPCs and the players who were fighting, but knew that I would die immediately as I was still under level 20 and all of the mob levels were unknown, although thankfully non-aggressive.
It was not until years later when the second Scourge Invasion occurred that I truly understood the scope of the event, but at the time, it was thrilling.
Too see a few more screenshots check out the gallery.
Were you playing when this event occurred? I have always wondered what it was like for level 60s at the time.
[All screenshots taken by Adele of Silver Hand on dates ranging from June 21, 2006 to July 08, 2006.]