The Gates of Ahn'Qiraj

Patch 1.9 saw the first true world event in the World of Warcraft. Located in the mysterious insect-infested, quasi-Egyptian themed area of Silithus, Ahn'Qiraj was the capital city of the powerful Qiraji, a race of magical creatures under the thrall of an ancient and terrible Old God who was chained beneath the earth in ages past. A coalition of Night Elves and dragons of the Four Flights fought a war against the Qiraji and their Silithid minions and sealed them behind the Scarab Wall. However, after many centuries the bonds of their prison began to break and Silithus was overrun by the creatures once more. A call for War against Ahn'Qiraj went out and the combined might of the Alliance and Horde sealed away the menace of the Old God for good.
The world event was triggered by a twofold action. First, the entire server population was able to take part in the Ahn'Qiraj War Effort. Players of every level could turn in various items in both Ironforge and Orgrimmar for their faction's respective war preparations. Metals, herbs, textiles, and other commodities were all collected in great quantities. For example, one collector in Ironforge required players to turn in a stack of 20 runecloth bandages at a time. The total number of runecloth bandages required numbered in the tens of thousands. The faster materials were turned in, the faster the War would commence. In the weeks leading up to the opening of the gates many servers were neck-and-neck as Blizzard provided a rankings page to monitor the progress of each realm. In the end the realm Medivh succeeded in being the first to open the gates of Ahn'Qiraj.
At the same time the War Effort was taking in supplies, high level players could engage in a quest chain that spanned the entire world to piece together an artifact called the Scepter of the Shifting Sands. This item would be necessary to ring the Scarab Gong and break the seals holding the Scarab Wall closed. When the War Effort was completed the armies of the Alliance and Horde would march to Ahn'Qiraj. In a spectacular set-piece the armies formed ranks outside the Scarab Wall and the gong was sounded by one lucky person per server. The gates opened and the minions of the Qiraji spilled out in a titanic melee. Additionally, invasions of Silithid insects occurred in almost every populated area of Azeroth.
With the initial event completed the gates were open to everyone on a given server and players were able to access two new raid dungeons: the 20-man ruins zone and the 40-man temple zone and a few new quests.