From WoWWiki

The Scarab Lord title is a reward for opening Ahn'Qiraj by completing the quest  [60] Bang a Gong!.

When the instances Ruins of Ahn'Qiraj and Temple of Ahn'Qiraj were added in patch 1.9, the Gates of Ahn'Qiraj were still closed and both instances could only be made accessible by completing a series of player-driven quests.

In addition to all players on a realm working towards completing the War Effort, one player had to create the Scepter of the Shifting Sands - a very difficult task that could only be solved by a server's top guild(s) back when the quest-line was added. After the War Effort was finished and the army of the alliance and the horde arrived in Silithus, the Scepter of the Shifting Sands could be used on the Scarab Gong. Doing so resulted in Ahn'Qiraj being opened. The first player to bang the Scarab Gong and anyone else who followed within 10 hours, mostly only 1-3 people each server, obtained the Scarab Lord title.

Although it is still possible to complete the quest-line for the Scepter of the Shifting Sands even when the gates are already open, doing so at a later time will not reward the quest-solver with the Black Qiraji Resonating Crystal or the Scarab Lord title.

See Also: Titles

See Also: List of scepter holders by server

Blizzard concerning titles

Nethaera:

First, we want these types of titles to be very unique and special in nature.

Second, we have a limited amount of slots in which to have titles available to people (thus why everyone can't just choose any PvP title they wish.) So, we want to continue to make titles available, have room to add new and unique titles and keep them as special as possible.

Overall, we intend to add titles to the system for various things where we can. Some will be very very specialized such as the Scarab Lord title and others will be more accessible via such things as the Arena system provided you earn it by climbing the ladder.

Titles
Arena	Challenger • Duelist • Gladiator • Rival
Battlegrounds	Conqueror • Justicar
Miscellaneous	Scarab Lord • Champion of the Naaru • Hand of A'dal • of the Shattered Sun