First, let’s look at how to actually upgrade your Legendary.
How to upgrade Legendary
Blizzard announced following
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:
Nighthold Raid Bosses
Weekly Mythic Keystone Cache
PvP Weekly Quests
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.
Now let´s look at how we can prepare ourselves for next week’s reset so that we will get the Essences more efficient.
How to upgrade Legendary faster
- Emissary World quest
It clearly says that you cannot save Emissary bags for next weeks reset in order to get Essences from them since it needs to be new bags.
But what it doesn’t say is that you can save the Emissary World quests 2 days before reset day. In this way, you will have quests to get 3 Emissary chests on reset day!
To save even more time on reset day, you can complete 3 out of 4 quests for the 2 emissary quests. In this way, you only have to complete one quest per emissary on reset day (except for the 3rd Emissary).
- Weekly Mythic Keystone Cache
You have to do a Mythic plus this week. It doesn’t matter which level you do (what we know for now). You will get the Essences from the weekly chest on reset day in your class order hall.
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