Since the release of Patch 7.1.5, we've received a number of questions relating to the Nighthold Legendary item upgrades and how that process will work. We'd like to provide some clarity on what you can expect next week.
When the Nighthold opens next week, players who have acquired at least one Legion Legendary item will find a quest in Dalaran to upgrade a Legendary from item level 910 to 940. This quest involves collecting a number of Essences of Aman’thul, which can be found in the following locations:
Weekly Mythic Keystone Cache
Once you have completed the quest, you'll be given a Distilled Titan Essence, which can be consumed to upgrade a Legion Legendary item to level 940. The quest is repeatable, so you'll be able to earn as many Distilled Titan Essences as you need to upgrade any Legendaries you have.
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.