After lighting the Lighthouse in The Brine Muskeg region in Graven, the next step of your journey has you returning to the depths underneath Cruxfirth. You’ll be entering the Quarantine zone where the plague hit the city the hardest in search of making your way through the numerous hordes of undead creatures and trying to find a way into the library. Along the way, you’ll find yourself attempting to solve a Pillar Puzzle.
The key to solving the Pillar Puzzle is finding two pieces of paper. You can find the first one in the barracks shortly after you gain a new gem for your staff. You’ll break down the doors, and on the small table next to the exit will be a piece of paper with these symbols. They’ll be important for later.
The second piece of paper with the other symbols will be inside a small house, in the lower area. From the barracks, go to the left, and continue going to the left until you reach the lower areas, full of undead. Take another left, and there’s a small building at the center, to the right of some stairs. Go inside, and there should be another paper with symbols. These ones will be different than the previous paper you found before.
You now have the two pieces of information you need to combine to complete a puzzle that’s right around the corner. Return the route you were going, but this time take a left. There will be a handful of undead enemies here, and when you go all the way to the left, there is a trio of levers you can pull to modify a large pillar. You need to match these two notes that you found as best as possible using the small details to glean off them both.
The goal is to match the symbols you discovered on the pieces of paper to the pillar. You don’t care about the middle symbols, only the ones on the left and right side when facing it. The pillar should look like this, and then you’ll receive the Turtle Key.
You now have access to the Turtle Key, which can be a little difficult to find. You want to look for an area full of water, and dive around to locate the door.