Hidden Potential of the Archdruid

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.

Balance
[item=139551] is obtained by combining two items. You need to be exalted with the Dreamweavers!
- [item=140652] can be bought from [npc=106901]
- [item=140653] has a chance to drop from [npc=96512] in Darkheart Thicket (any difficulty)

Feral
Feral hidden skin is unlocked using [item=139552].
Every day when you visit Emerald Dreamway, you have a chance to see an emote that says "You hear a screech coming from the Hinterlands/Duskwood/Feralas portal." If you see this emote, go into the specific portal mentioned in the emote and look for a grey owlcat stone. It's in a different location every time. When you find the stone, click it, an emote will appear in the chat "You hear something dart away behind you... and then just silence." and you will get credit for the stone.
After you touch all three owlcat stones in all three zones, you can return to Emerald Dreamway where you'll find Ela'lothen sitting in the middle of the zone. /sit in front of him and you will receive the [item=139552]

Guardian
[item=139553] has a chance to drop from [npc=100497] in The Emerald Nightmare (any difficulty)

Restoration
[item=139554] has a chance to be looted from the Evergreen plots of rich soil.
In your class hall upgrades ([npc=97989]), research [order-advancement=354] (second row, left side). This research will allow you to plant two seeds in your class hall, which have a chance to drop the hidden appearance. [object=253078] can be found at /way 45.2 63.5

https://warcraft-secrets.com/guides/druid-hidden-artifacts-appearances-and-effects