With the release of The Nighthold Raid, Legendary items can now have a item level of 940. Here is a quick guide on the upgrade process.
Once The Nighthold is finally playable, every new Legendary that drops will already have item level 940. The ones you already have in your bags can be upgrade with a rather simple quest.
Upgrade Your Existing Legendaries To 940
- Nighthold Raid Bosses (per-boss basis, will drop on all difficulties!)
- Weekly Mythic Keystone Cache
- Emissary Bags
- PvP Weekly Quests
TL;DR: PICK UP QUEST BEFORE OPENING YOUR WEEKLY CHEST!
The weekly chest will contain 15 and an emissary Bag will reward you 2-4 Essences (random). This means you probably will be able to upgrade two of your Legendaries until you enter The Nighthold on Mythic.
Once you have collected 50 Essences, hand in the Quest and you get Distilled Titan Essence as a reward. Use this to upgrade a Legendary item up to item level 940.
Repeat this process for every single legendary you have.
No Further Gating
This upgrade process for Legendaries is not gated behind or tied to the Illidan quests. Done!