After questing a bit for the Nightfallen in Suramar, you will eventually unlock a world quest that shows up roughly every three days. This allows you to play through a solo scenario in which you lead forces through the ruins of Falanaar.
Each time you show up, you'll want to bring at least 400 Ancient Mana. I recommend going in with 500, because if you aren't keeping up with Thalyssra, she'll probably need to be fed so she can interact with you. The extra 50 I recommend bringing for Valtrois, because her buff both is very helpful in the scenario and also helps you recoup some if not all of the Ancient Mana spent (in fact, you might even make a profit.)
So why do this scenario?
First off, you'll get gear (though its quality drops off a bit if you're decently geared in other ways.) You'll also get lots of toys and reputation with the Nightfallen - I'm closest to Exalted on them thanks in large part to this scenario.
Additionally, and frankly my main motivation, is that Protection Paladins, Blood Death Knights, and Windwalker Monks have a chance (it's small - I've done this every available time on my Paladin for about a month and have yet to see it) to get the hidden artifact appearance for their spec out of the chest you receive at the end of the scenario.
The chest at the end gives more reputation the higher you score, and you get a higher score by killing enemies inside the scenario. I've done a couple of full clears and still have yet to get the hidden appearance, so don't hold your breath.
So how does it work?
Based on the amount of Ancient Mana you bring, you'll get a number of Withered Exiles who will follow you around. The minimum is 400 Ancient Mana which gives you 8 Withered. These guys will follow you and fight alongside you. You can command them to go Berserk, increasing their damage and having them attack everything in sight, or you can have them simply follow you and attack things you attack. I find it's easier to keep them alive if I keep them on the Follow setting (which is the default.)
Withered cannot be healed, so you really need to play carefully to avoid getting them killed. Using stuns, knockbacks, and interrupts will be very important when dealing with a number of enemies.
Over the course of the scenario, you will find many friendly Withered who can be recruited by right-clicking them. They will be added to your troops and follow you like the ones you started with. Starting with only 8, I've managed to amass at one point 40 Withered, though I would not hold out expectations for this kind of army on your first run.
You will want to keep as many Withered with you as possible for many reasons. First, they will attack your enemies, and when you get a sizable army they do a massive amount of damage.
Second, there are several doors that require at least 10 Withered in your army to open. You will find tough enemies as well as important treasures (we'll get to those next) in these rooms. There is also one door that requires two Withered Berserkers, which we'll get to below.
Third, there are several chests to be found around the scenario. Small chests can simply be opened and will spawn randomly, even in the middle of the scenario, though these usually don't have more than gold and a bit of Ancient Mana. However, there are larger chests that you only need to loot once (ever, not just in that particular scenario.) These chests will contain toys as well as items that enhance your Withered Army the next time your run the scenario. However, in order to open these, you have to send a number of your Withered, either 5 or 10 depending on the chest, to haul it back, meaning that your army will shrink.
I highly, highly recommend grabbing any chest you can in this manner, as many of them contain items that will either increase the health or damage of your Withered or they will give a member of your initial group a set of special abilities, transforming them into Starcallers, Mana-Ragers, or Berserkers, which vastly increases their damage and give them new abilities (Berserkers also taunt targets, though I always taunt back as they get themselves killed pretty quickly if they're taking the enemies' attacks.)
Strategically, I recommend picking up chests and also opening any doors you can with your 10 Withered but not actually going in until you have opened all the doors. There is one door early on that requires two Berserkers (and that room is a positive abattoir) and another in the initial temple building that requires 10 of any Withered. Farther into the ruins, there are two doors that require 10 that lead to "bosses" (just tough enemies with names) and another that leads to a big chest and three friendly Withered.
Once you get into the large central chamber, you will - at a random time - get attacked by a Nightborne assassin named Leystalker Dro. Killing him will give you a key that is needed to progress further, through a door that you would have passed going to the other "boss" rooms.
Going through this locked room, you will find a chamber with several eggs. Upon clearing the Fal'dorei from there, a door opposite the room you just came in will open up (that previously appeared to be non-interactive) and you will be able to fight Elfbreaker, an Ettin who is probably the toughest of the bosses. You'll also get a constant stream of little spiders until you follow a set of stairs up to the final boss, who is a large spider.
One key thing for this scenario is that you're not expected to make it through the first time. Think of it as a progression, and try to find and get those big chests each time, as it will make the next time go smoother. At this point, I've been able to finish the scenario with a 30-strong Withered Army, but it took me many, many attempts to get to the end.