Mer-Ky Depths is a Hogwarts Legacy side quest in which you help Nerida Roberts get a step closer to becoming the “Liaison to the Merpeople.” Nerida gave the Merpeople a gift, and they wanted to leave one for her in return in an underwater cave.
The problem is that the future liaison to the Merpeople doesn’t know how to swim and can’t get to the cave. This is where you come in and retrieve the gift for Nerida by getting into the cave and solving a little puzzle. As a reward, you’ll receive a Mermish Liaison Uniform and 180 XP.
To unlock Mer-Ky Depths, you must first complete the Percival Rackham’s Trial quest. After that, you must find Nerida Roberts, located near the Boathouse in the south part of Hogwarts Castle.
To get to Nerida easily, open your map, select the Great Hall area, select the side quest icon near the Boathouse, and press X/A on your controller. Now, all you must do to reach the Boathouse is pick your closest unlocked Floo Flame and fast-travel there. Then, make your way to the quest giver waiting by the water, like in the next image.
Where To Find the Leaky Caves
After talking to Nerida and starting the Hogwarts Legacy Mer-Ky Depths quest, you must find a way inside the Leaky Caves, where the Merpeople placed their gift. If you look in front of the NPC at some distance, you should see a marker in the water.
Swim to the marker, and when you reach it, press Square/X to dive.
Where To Find the Mermish Artefact
You’ll now find yourself in a cave with the quest tracker leading you to a closed door. The door has three butterfly-shaped holes on it, which you need to fill.
If you explore the small cave, you’ll find the three butterflies needed to solve the Mer-Ky Depths puzzle: one to the south, another to the west, and the last to the north. You’ll find the insects easily because the cave is really small.
If you go near the western butterfly and cast the Lumos spell, the insect will follow you. So, lead it to the closed door. When you get close to the door, press the button assigned to Lumos again, and the butterfly will get in one of the holes.
You have to do the same with the other two butterflies, but the problem is that they are both behind some spider webs. To remove the spider web, you must burn it by casting Incendio, which you’ve learned during Professor Hecat’s Assignment 1. Once you guide the third butterfly to the locked door, it will open, solving the puzzle and revealing the Mermish Artifact chest.
Now dive back in the water, return to Nerida, and give her the gift, completing the Hogwarts Legacy Mer-Ky Depths side quest.