Hidden Potential of the Shadowblade

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.

Assassination
[item=139570] has a chance to drop from chests and elite mobs in The Underbelly.

Outlaw
To get [item=139536] you need to:
- get [item=139466] that drops from [npc=91007] in Neltharion's Lair (any difficulty)
- get [item=139468] that drops from [npc=95886] in Vault of the Wardens (any difficulty)
- get/buy 10x Infernal Brimstone
After you get all 3 items, go to Stormheim and find Thrymjaris /way #634 40.8 80.8 Thrymjaris
Switch to outlaw spec and talk with her, select the second option and continue with the conversation until you pick up quest Bindings of the Windlord. Complete it and pick up the next one Audience with the Windlord, which will award you the hidden appearance.
If you select the first option in the dialog by mistake, you will receive a debuff that will stop you from obtaining the hidden skin today. Either come back tomorrow, or queue for arena skirmish (doing that will make the debuff go away) and you can talk with her again.
IF YOU DON'T SEE THRYMJARIS, you need to, unfortunately, progress with the Stormheim storyline. Go to your class hall and on the scouting map select Stormheim. Continue with the campaign quests until the point where you meet the dragons on top of the mountain, Where Dragons Rule. Doing the quests from the start takes around 20 minutes.

Subtlety
[item=139571] has a chance to drop from Lei Shen's Burial Trove chests in Troves of the Thunder King scenario.
In your class hall upgrades ([npc=105998]) research [order-advancement=441] (second row, left option). Completing missions will give you a chance to receive [item=140774]. You can then give the [item=140774] to [npc=102594] and you will recieve [item=139781] (you can also get it by pickpocketing in the Broken Isles). For the [item=139781] you can pick something from the vault - one of the options is the [item=94222]. Keep in mind, that treasures rotate, so the keys might not be available every day. Once you have the keys, speak with [npc=98100] to start the [zone=6716] scenario. Simply complete the scenario (by reaching the end, or until the timer runs out), loot chests on the way, kills enemies and obtain as many [item=95497] as possible. At the end of the scenario, you can open [object=218949], which can contain [item=139571].

Thanks so much about the Lemix info, I had a low level rogue (level 20) so I picked up the quest to do the Subtlety artifact and once I had it, I then changed spec, equipped the fangs, transferred the bronze from my main and then bought the tome :)