Timeless Essence of the Black Dragonflight

The Timeless Essence of the Black Dragonflight, awarded by Wrathion for completing the Patch 5.4-portion of his legendary quest, will take any Cloak of Virtue and upgrade it to a  cloak (a Shroud of the Celestials) with the same stats but a new, quite powerful proc. The legendary cloaks will automatically be fully upgraded to item level 608.

Source

 * A Pandaren Legend

Replacements can be purchased for from the Blacktalon Quartermaster in the Tavern in the Mists after completing A Pandaren Legend.