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 was 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 would not reward the quest-solver with the Black Qiraji Resonating Crystal or the Scarab Lord title.
In lore, it is known that Shiromar is the Scarab Lord rather than a random player.
Siege Engineer Quarterflash mentions the Scarab Lord when talking about heroes that are fighting alongside the 7th Legion in Northrend. This in context really refers to the player that received the title, but still might be Shiromar.