Today's PTR update includes a new method for acquiring Legendary items: the Purified Titan Essence. This item can be purchased from Arcanomancer Vridiel in Dalaran in exchange for 175 Wakening Essences.
When used, the Purified Titan Essence will immediately grant a Legion Legendary item appropriate for your specialization. Note that this item is bind on pickup, and that the Legendary item granted by the Purified Titan Essence does not affect your chance to earn a Legendary through other methods.
Additionally, the Awoken Titan Essence used to upgrade Legendary items to item level 1000 is now sold by Arcanomancer Vridiel as a vendor item, rather than as a quest reward, for the same total cost (50 Wakening Essences). The quest will no longer be offered, but can be turned in if your character already has it active.