Secrets of the Empire

Source
Secrets of the Empire are a semi-rare drop from all bosses in the Throne of Thunder or the eight first bosses in Siege of Orgrimmar in all difficulty levels. The chance for the item to drop is one attempt per eligible boss, per week across all difficulties, and the chance for the item to drop is the same regardless of difficulty mode. Lei Shen and the Sha of Pride have a 100% drop rate.

Much like with the Sigil of Power and Sigil of Wisdom back in Patch 5.0, in all difficulty modes, secrets are personal loot, meaning that each member of the raid has an individual chance to loot one copy. Additionally, players only have one chance at a secret per boss, no matter the difficulty mode. Thus, if players kill a boss in Raid Finder, then later turn around and defeat it in Normal or Heroic mode, only the Raid Finder kill will be eligible to drop a quest item.

As a quest objective

 * Secrets of the First Empire