The Emerald Nightmare is the first raid in patch 7.0, the raid including seven bosses which were divided into four difficult: LFR, Normal, Heroic EN, and Mythic, its'drop gears are at ilvl835-880. Clearing Mythic EN will get Mythic Achievements for all bosses, also will gain Feats of Strength: Ahead of the Curve: Xavius and Cutting Edge: Xavius.
Players get wow gears basically at ilvl880 in Mythic The Emerald Nightmare, also have small chances to take Titanforged items in Legion raid. Titanforged use to mark the upgrade of an item is greater than or equal to 15 levels, it changes with the highest ilvl item in the current patch. It is to say, for example, the highest item in Legion is at ilvl910, so players have an opportunity to upgrade any gears in Mythic EN to ilvl910.
Mythic The Emerald Nightmare requires 20 men and it drops very limited gears, especially the necklaces, rings, cloaks, and relics may cause a rat race. But, Seal of Broken Fate will help improve, it allows you have a bonus loot time in raid or world boss. To avoid bad luck, you are sure to gain a gear after using a certain number of Seal of Broken Fate. You can purchase Seal of Broken Fate with Order Resources and WOW Gold btw, and you can only get three seals a week. 
Just prepare enough Seal of Broken Fate, you may get more than ten pieces of gears in Mythic EN raid if good luck, it is also the quickest way to upgrade your character's ilvl.