Hello Fellow Steam Achievement hunters,
Here's what I did to unlock this achievement...
Roll The Forgotten Kingdom dlc on whatever difficulty works for you. Make sure the Infested Abyss dungeon is in your roll, it's near the start. Note, you'll also be able to get the Crystal Staff melee weapon in this dungeon. Once in Infested Abyss you'll have to defeat three root nexus to progress through the dungeon. Don't fall of ledges exploring like I did lol. Be advised during the 3rd root nexus encounter you'll face Venom and a few other larger enemies. I used an elemental tank build and it got me through on apocalypse without a hitch. Finally, you'll encounter The Emissary boss, DONT FIGHT HIM, instead go through the dialog and accept his task. He will then gift you a corrupted Soul Stone amulet called the Profane Soul Stone. You need this amulet to get the achievement.
With the newly acquired amulet head back to the ward and then on to the labyrinth in your campaign roll and find The Keeper. When you warp to the labyrinth make sure you are wearing the Profane Soul Stone amulet. For me, The Keeper was easy to find where you originally spoke to Clementine. Engage in the dialog with the Keeper until all questions are exhausted. Then they will mention you are wearing a corrupted entity and offer to cleanse it for you. Give the amulet to The Keeper and they will return it cleansed as the Soul Stone amulet and BOOM the achievement will be unlocked.
If the Keeper isn't in the labyrinth for whatever reason. You can also find them in Root Earth at the entrance of the final boss. If the Keeper is nowhere to be found in your campaign, which would suck, then reroll your campaign and progress until you reach the labyrinth where you speak to the Keeper and Clementine for the first time and repeat the step above. Hopefully you don't have to go through all this and it is as easy to unlock this achievement as it was for me.
Happy Hunting and Good luck!!