This achievement is earned after completing the main quest called "The Trial of the Waters". This is story related and can't be missed. Spoilers ahead.
Note: A sleeping potion is highly suggested for this quest.
You will be sent to the village Dorhadgenedu in Dorgred, "The Heart's Gates" to speak with Glendan. He will tell you of a trial that you must undertake in a cave behind the village.
Head up to the cave and enter to start the trial. Head a little bit up the path to see some Ulgs feasting on a carcass. Wait for them to move away from it and sneak over to it to use the sleeping potion on it. This will put all the beasts to sleep and allow you to pass quietly and peacefully. Proceed to the next section of the cave.
Here there will be two paths you can take. If you have Vigor 2 you can take the path on the left. If you don't have the required Vigor, then you must take the path on the right. The path on the right has a ton of Dosantats hanging from the ceiling. You can click in
to sneak past all of them though and proceed to the next section of the cave.
Here there will be a pool of water in the middle of the room and off to the left will be a Beach Guardian. DO NOT fight the Guardian. Just ignore it for now. Examine the pool in the middle and De Sardet will speak about a vision he had. Pay attention to how he describes the vision as you will need to light candles on nearby stones with markings in the order that matches the vision.
The correct order of stones / candles is:
Once you lit all the candles in the correct order you should see that the Guardian is now dormant. Sneak over to the Guardian and get within melee distance, you should see a prompt with
to tame the Guardian. Doing so will open the large door behind it. Head through the door and to the left to inspect a mural on the wall. Now you will be teleported outside. Head back to Glendan and speak to him. After the conversation is over the achievement will pop.