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Ruins of Ahn'Qiraj
Defeat Ossirian the Unscarred.
The 'Ruins of Ahn'Qiraj' achievement is earned by defeating Ossirian the Unscarred, the final boss of the AQ20 raid located in Silithus.
Enter the Raid
Enter the Ruins of Ahn'Qiraj and defeat the first required boss, Kurinnaxx. He is a straightforward fight but applies a debuff that reduces your chance to hit by 75%, so be aware of that during the encounter.
Reach Ossirian the Unscarred
You only need to kill two bosses before Ossirian — the remaining bosses are optional. Make your way to Ossirian's room and clear all the adds in the area before pulling him so they do not interfere with the fight.
Defeat Ossirian the Unscarred
Pull Ossirian and immediately run to the nearest crystal sticking out of the ground and right-click it to activate it, which places a debuff on him making him vulnerable to a specific type of magic. Keep activating crystals as you move around the room until he becomes vulnerable to your damage type, then burn him down to complete the achievement and loot the 
Head of Ossirian the Unscarred.
Tips
If your bags fill up while farming Ruins of Ahn'Qiraj, the Oasis of Vir'sar is the closest vendor and mailbox location.