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This gem is rewarded for completing the first phase of the legendary quest chain. Agility, Strength and Intellect are the three options you may choose from. These gems can only be placed in the sockets of a Sha-Touched weapon.

If you replace the weapon you have placed your gem in, you can purchase a new gem from Commander Oxheart, the Valor vendor in Townlong Steppes for 10,000 gold (guild discount will apply).

The full quest chain to earn this gem is as follows:

The full path to the legendary gem begins with the Mysterious Note, which is given from several NPCs in Pandaria.  It will direct to you visit Wrathion in the Veiled Stair for the quest Stranger in a Strange Land.  He will ask you to share a drink with him for the quest A Legend in the Making which gives you some lore background and his story. 

After this, Wrathion will then give you two quests to complete:
•  The Strength of One's Foes - Sigils of Power and Sigils of Wisdom drop in Tier 14 raid instances, in any difficulty level.
•  Trial of the Black Prince -  The Reputation can be farmed from various level 90 mobs out in the world.

After these two quests are completed, you will get the quest Fear Itself which requires you to kill the Sha of Fear in the Terrace of the Endless Spring to get the Chimera of Fear.  This item can be looted in LFR, Normal or Heroic modes.

Finally, he will ask you to meet just outside the inn to complete Breath of the Black Prince and pick up your new Legendary gem.