Withered Army Training is a World Quest Scenario in Suramar. It will reset in few days. The mission will cost 800+ Ancient Mana to start a Scenario to do Withered Army Training, similar to achievement In the Hall of the Thunder King.
You will gain reputation with the Nightfallen through the Scenario, then use the rep to unlock Mythic: The Arcway and Mythic: Court of Stars, Class Hall set, and Broken Isles Pathfinder Part One. Withered Army Training is important in early Legion.
The Scenario will start in Falanaar, the first time you get the quest you are given a team of withered(at least 8, up to 20) according to the Ancient Mana you've turned. You can upgrade your upper limit of Ancient Mana by finishing quests so that to reward more Withered. In Scenario, you need to protect your Withered and control them to defeat the bosses and gain reward chests. The higher score, the more reward. You would transmit to the gate if you Withered all died. Fortunately, a few of the withered you encounter are non-hostile and can be added to your army.
There are many chests in Scenario, need your Withered carry the chests back. Medium chests need 10 Withered, while Small chests need 5 Withered. At the same time, a Large chest will be given when finished Scenario. In the Large chest, you may reward pets, toys, reputation badge, Artifact power, Blood of Sargeras. A rare chance at Torn Invitation, the quest item to obtain Reins of the Llothien Prowler. Some classes will also get their hidden Artifact appearance. Some purple and blue items you reward use to increase the capacity of Withered permanently, it makes the future training easier!
Tank spec or high ilvl will help you easily gain a higher score.