The first thing you'll need to do is obtain "Candrael's Charm" from a random drop - I assume it comes from Suramar guards, because that's where I got mine. No idea if that's the case. [Note: Other posts in the thread indicate that it is a zone drop.]
Once you have that, you'll notice the gem has "C.T." engraved on it. This stands for Candrael Twinshadow. If you go into the Felsoul Hold crater area in Suramar, there's a path visible on the map that runs west. Go up the incline and double-jump/glide along the rocks to get to the top of the cliffside. Follow it around to a large tree and you'll find Candrael Twinshadow, a Blood Elf Demon Hunter. She's by a large tree, some bird/porcupine mobs just before her.
If you show her the charm, her immediate reaction is to assume you're a thief and kick you off the side of the cliff - ya know, as you do. Her kick will send you straight at "Downfall," a large felbat with around 10 million health. You may've noticed him floating around in the general questing area. There's "violent winds" around him which will keep you aloft. You basically can "swim" in the spiral around him, but can't go too high or low or else you will drop out of the area and will reset (you can go to Candrael to continue attempting it). After damaging him for a little bit, Downfall will fling you off in a direction. You basically need to stay afloat using Glide and fall back on top of him to get kicked up by his winds again.
From my attempts (took around 5-10 to realize this was a DH only encounter and I had to be creative) it seems like you need to cancel Glide once you get back to his winds or it'll be buggy and let you fall right through. He'll fling you around again randomly even when you get back into his "bubble" of wind.
Kill him and you get the Guise of the Deathwalker.
Right now there are two ways to get Guise of the Deathwalker, the right way and the easy one. I believe you have to be Havoc to get the item.
The right way: 1. Farm mobs in Suramar, they have a chance to drop Candrael's Charm. It may take a long time before you get the item. 2. Bring it to Candrael Twinshadow, she's near Felsoul Hold on the top of the hill, 28.35, 70.3 3. She'll launch you at Downfall which has a special wind aura that let's you fly near him. You have to kill him 4. While fighting he'll knock you away from time to time, you have to use Glide and fly back to him or you'll fall. So to be safe you have to position yourself higher relative to Downfall so when he knocks you away you have more space to fall. He drops Guise of the Deathwalker, which gives you havoc artifact hidden appearance and an achievement Hidden Potential
/way Suramar 28.35, 70.3 Candrael Twinshadow
The easy way:
1. You have to go on the top of the Ironhorn Enclave flight point mountain in Highmountain. Start your way from Thunder Totem's second floor and go to the direction of Snowblind Messa (49.9, 63.6), when you reach the bridge turn right. Go along the road and you'll see Neltharion's Lair entrance. There is a path up the mountain near the dungeon entrance, take the path and you'll get up the mountain. 2. After passing two bridges you'll get on top of the mountain at 56.3, 94.1. From there you have to jump in Felsoul Hold direction using your Glide ability. Glide right into Downfall position and kill him using strategy from previous way. 3. Profit Guise of the Deathwalker
/way Highmountain 48.2, 68.3 road up the mountain (DH Artifact) /way Highmountain 56.3, 94.1 rjump from here to Felsoul (DH Artifact) /way Suramar 31, 71 Downfall (DH Artifact)
Edit: There was another requirement added when Legion went live. Your demon hunter must now also have a minimum Artifact Knowledge of Rank 5.
This item unlocks the hidden appearance called Deathwalker for the Havoc demon hunter artifact, Twinblades of the Deceiver.
From Azarakel via MMO-Champion:
Right now there are two ways to get Guise of the Deathwalker, the right way and the easy one. I believe you have to be Havoc to get the item.
The right way:
1. Farm mobs in Suramar, they have a chance to drop Candrael's Charm. It may take a long time before you get the item.
2. Bring it to Candrael Twinshadow, she's near Felsoul Hold on the top of the hill, 28.35, 70.3
3. She'll launch you at Downfall which has a special wind aura that let's you fly near him. You have to kill him
4. While fighting he'll knock you away from time to time, you have to use Glide and fly back to him or you'll fall. So to be safe you have to position yourself higher relative to Downfall so when he knocks you away you have more space to fall. He drops Guise of the Deathwalker, which gives you havoc artifact hidden appearance and an achievement Hidden Potential
The easy way:
1. You have to go on the top of the Ironhorn Enclave flight point mountain in Highmountain. Start your way from Thunder Totem's second floor and go to the direction of Snowblind Messa (49.9, 63.6), when you reach the bridge turn right. Go along the road and you'll see Neltharion's Lair entrance. There is a path up the mountain near the dungeon entrance, take the path and you'll get up the mountain.
2. After passing two bridges you'll get on top of the mountain at 56.3, 94.1. From there you have to jump in Felsoul Hold direction using your Glide ability. Glide right into Downfall position and kill him using strategy from previous way.
3. Profit Guise of the Deathwalker
/way Highmountain 56.3, 94.1 rjump from here to Felsoul (DH Artifact)
/way Suramar 31, 71 Downfall (DH Artifact)
Candrael Twinshadow and Downfall location