Hidden Potential of the Deathlord

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.

Blood
[item=139546] 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.

Frost
[item=139547] can drop from The Soultakers (Broken Isles World Boss).

Unholy
When you summon Army of the Dead [spell=275699], you have a chance to summon a special minion, which can drop [object=252302]. When you click the journal, it will tell you about Putricide's Secret Lab, which can be accessed by a secret door in Icecrown Citadel. Go to the raid and to Professor Putricide's room, kill him and you will see a trap door on the ground. Click it and you will be ported into a secret room, where you can find [npc=111093] - kill it and he will drop [item=139548]

To do achievement this as Unholy DK is quite simple but can take a lot of time. (After trying myself I suggest researching on how to do it as another spec but if you are stubborn here we go)

Army of the dead & Apocalypse abilities have a small chance (Probably 1%) to spawn a special NPC and you are required to spawn this NPC before moving on to the next step. To do this I suggest you to enter proving grounds where you can keep reseting your cooldowns. More info below on how to get there.

Use Deathgate to get to your order hall and speak with [npc=93491] to enter proving grounds.

Talk to the trial master to start basic damage fight on Bronze difficulty and simply cast "Army of the dead" then apply festering strike 2 times and use "Apocalypse" on the first mob that spawns. Make sure you are specced out of "Legion of souls" or "Raise Abomination" as these talents replace army of the dead.

After using these two cooldown abilities go back to the trial master to yield the fight and start over.

Keep using this macro after every attempt to know if the npc has spawned: /script print(C_QuestLog.IsQuestFlaggedCompleted(44188))

if it says false keep on repeating this process

Once you have made the NPC spawn simply head to the Icecrown Citadel raid in Northrend and go to professor Putricide (boss) and on the right side will be a teleport pad you can use to enter another secret room with a npc dropping your hidden artifact appearance

It helps to run /run print(C_QuestLog.IsQuestFlaggedCompleted(44312)) to see if you already found the lost journal. It doesn't go in your bags. Once I realized I had already found it, the rest was a piece of cake.