Under the frigid expanses of Dragonblight, a dark secret is kept. Sartharion with his three lieutenants keep vigil over a clutch of twilight eggs. The dark future that these eggs could set in motion could wreck havoc all across Azeroth. To learn how to defeat Sartharion and about the loot and achievements related to this instance, look here.
The Obsidian Sanctum allows for 10 or 25 man versions of the raid, and also allows groups of raiders to decide if they would like to kill all three drakes, or leave some combination of them alive. Defeating Sartharion with one or more drakes up increases the difficulty but in turn improves the loot drops.
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