Hidden Potential of the Slayer

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.

Havoc
Farm mobs in Suramar until [item=141409] drops. It can drop from any mob in the zone - a really good spot is near [npc=111649], there are many mobs in this area and they respawn fast.
Once you have the [item=141409], bring it to [npc=113935] - /way #680 28.4 70.2 Candrael Twinshadow
Talk to her and she will kick you into the air towards [npc=111110]. Kill him and loot [item=139549] from the corpse.

Vengeance
[item=139550] has a daily chance to drop from bosses you kill from the [order-advancement=419] portal.
To access the Twisting Nether, you need to research it with your class hall upgrades. Speak with Loramus Thalipedes (downstairs in the Fel Hammer), [order-advancement=419] is in the 2nd row, left side. Once the research is complete, a crystal will appear next to Loramus Thalipedes. Click on it to enter, and while inside, click the Fel Tome located between two Priestesses. This will spawn a demon from the Twisting Nether, that has a chance to drop the hidden appearance.

Ambassador D'vwinn location worked perfectly. Charm dropped after about 2 mins of farming

If you going for the Havoc wep on retail with a DH you played in legion, CHECK YOUR BANK TO SEE IF YOU HAVE THE CHARM. Since it is not very clear what you needed to do with this item you may have banked it 10 years ago like me. I was farming for a good two hours with another person and noticed I was getting "there is no loot" errors. Lo and behold I check my bank and see it right there.