In World of Warcraft: Legion every artifact weapon has a hidden appearance. Although it has been determined how to obtain most of them, the Havoc Demon Hunter's Deathwalker is shrouded in secrecy.
In order to obtain the Deathwalker you will first need to find Candrael's Charm. This is a very rare drop form World Quests in Suramar.
Once you have Candrael's Charm, bring it to Candrael Twinshadow near Felsoul Hold. You will then be introduced to Downfall, an enemy boss you will have to solo.
To beat Downfall, be sure to use Glide to hop between winds and avoid hitting the ground. Upon killing the boss you will obtain Guise of the Deathwalker, which upon use will unlock Deathwalker.
If you're interested in obtaining the Vengeance Demon Hunter hidden artifact appearance, Iron Warden, simply make sure you have the Twisting Nether hall trait which allows you to portal into the Twisting Nether and battle a boss. The boss has a low percentage chance of dropping Bulwark of the Iron Warden, which upon use will unlock Iron Warden.

Embark on an inspiring mission of human connection beyond the UCA. Sam — with companions by his side — sets out on a new journey to save humanity from extinction. Join them as they traverse a world beset by otherworldly enemies, obstacles and a haunting question: should we have connected?