The World of Warcraft community site has been updated with information about how players will be able to upgrade current iLevel 910 legendary items to iLevel 940 once The Nighthold raid goes live next week. Those with legendaries will be able to pick up a repeatable quest in Dalaran that sends them out to “collect a number of Essences of Aman’thul”, though the precise number was not revealed. Once done, a Distilled Titan Essence will be awarded that will raise the legendary iLevel.
Essences can drop in a number of ways including from Nighthold raid bosses, Mythic + weekly reward chests, Emissary reward chests and in weekly PvP reward chests.
Additionally, once the Nighthold is open, any new Legendaries you discover will already be item level 940. Note that this does not apply retroactively to Emissary Bags that you may already have in your inventory when the time comes. Only new bags earned after the Nighthold is open have the chance to reward an item level 940 Legendary.
Read more at http://www.mmorpg.com/world-of-warcraft/news/legendary-upgrade-to-940-via-quest-to-collect-1000042797#9uoWT40A3vuwOwDg.99