This quest is not available in game.
Recover the Fragment of Spite from Blackrook Hold (Mythic), the Fragment of Power from Violet Hold (Mythic), and the Fragment of Torment from Vault of the Wardens (Mythic).
It would seem secret orders have been parceled out by the Legion's masters. If they're determined to conceal their plan from us, it must pose a dire threat.
We know high-ranking demons and their allies are mobilizing in places of power. Odds are good that they have received similar crystals.
Perhaps you can find one intact, or at least gather enough fragments that we can reassemble one and determine the Legion's target.