Hidden Potential of the Highlord

To complete this achievement, you need to unlock one hidden appearance for your class. To be able to get a hidden appearance, you need to have the class hall unlocked and the base artifact for your spec.

Holy
[item=139564] can drop from demons in Broken Isles and Argus.

Protection
[item=139565] has a chance to drop from the Withered Army Training. It is speculated that you need to reach a score above 300 to have the chance to get the appearance.
To unlock Withered Army Training, you need to do the Suramar campaign until you get a quest Building an Army, and after you complete it, you will have Withered Army Training unlocked.

Retribution
To obtain [item=139566] you need to:

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• Get two books:
  - Nat Pagle's Guide to Extreme Anglin', looted from players in Alterac Valley. Alternatively, you can buy it on the auction house.
  - A Thoroughly Read Copy of "Nat Pagle's Guide to Extreme Anglin'.", looted from [object=179547] in [zone=2557]. Alternatively, you can buy it on the auction house.
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• Kill [npc=11583] in [zone=2677] and loot [item=19003]/[item=19002]. DO NOT turn in the quest.
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• Talk with [npc=91190] with both books. You need to be in retribution spec. She will give you [item=139620]
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• Talk with [npc=11486] in [zone=2557] West with the new book and Nefarian's head. Do not kill [npc=11496]
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• Talk with
  If you are alliance: [npc=11609] in Western Plaguelands /way #22 43.0 84.2 Alexia Ironknife
  If you are horde: [npc=11608] in Tirisfal Glades /way #18 83.2 71.8 Bardu Sharpeye
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• Read [npc=111121] at Hearthglen in Western Plaguelands /way #22 42.36 18.73 Grand Inquisitor Isillien's Journal
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• Kill [npc=111122] and loot [item=139623] /way #22 69.0 45.8 Large Vile Slime
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• Use [item=139623] and talk to [npc=111123]
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• Fish up [item=139624] from Thondroril River /way #22 69.8 46.0
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• Talk to [npc=90259] in your class hall with the shard and accept Walking in Shadows.
  If Lord Maxwell is not there, you probably sent him on a mission, so wait until he's back
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• Use Dalaran flight master to travel to the Acherus: The Ebon Hold, where you need to complete a short event.
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• After that, return to [npc=90259] in your class hall, turn in the quest and you will receive [item=139566].
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You can buy Shard of Darkness from Horos in the Infinite Bazaar during Legion Remix.

In Remix you can just buy the Shard of Darkness with Bronze and save yourself the trouble. The guy with the mounts sells them.