This is a fairly brief dungeon located in Liurnia of the Lakes. As you’d expect from the name, it’s home to plenty of crystals — heck, even the bosses are crystal! It can be pretty dark at the start, so consider bringing a Lantern or other light source. Note that this dungeon requires two Stonesword Keys to enter, but you find one once inside.
From the site of grace, head down into the dark (east) and past the summoning pool. You’ll be forced to turn right, at which point you should use the target lock to find the closest rat. There’s one rat nearby, and then two more around the next corner, just past the rock that sits in the middle of the path — There’s plenty of Crystal Cave Moss to grab around here too, so make sure you grab it.
Once you’ve laid the rats to rest, continue forward through the tunnel, and you’ll eventually come to a more open area that’s shrouded in mist. Turn right and use your lock on, and you should see a dark figure just ahead of you. If you have some AOE options, like a specific spell or firebombs, get one ready — you’ll need it in a bit.
When you’re ready, sneak up and backstab him — this will aggro another mage just ahead of you around the crystal (to your north). Run around and deal with him, then hide behind the crystal in front of you to the north and wait a moment. A trio of sorcerers will be coming around the corner (though every once and while they might stay put, in which case you’ll need to go to them). Either way, use your AOE on them — if you’re a melee user, a leaping heavy attack should hit two or all three of them if you time it right. Once they’re down, turn around and go south-southeast through the room. In the corner there, you’ll find x5 Cuckoo Glinstone.
Now turn around, hug the left-hand wall, and continue north. You’ll pass the tunnel you came through — keep going, and you’ll soon run into yet another sorcerer. From where the sorcerer was standing, turn left (west) and head down the passage. You’ll enter a room with a long-dead prisoner — grab the Stonesword Key from them and go back the way you came. As you exit the passage, you’ll see a lit doorway across the room from you, which is the way forward. There are a number of Silver Fireflies in an alcove on your left that you can grab before you continue.
Once you’re done bug-collecting, head through the tunnel. You’ll soon reach a wooden door — head through and start sneaking. You’ll see a huge sorcerer standing in a doorway, and one more patrolling — ignore him for now and look left, where you can roll into an illusory wall.
Head up the newly opened path, and backstab or otherwise dispatch the sorcerer you find just up the path. This will bring the big boy sorcerer around the corner; greet him with a leaping attack or a charged spell, and then try to dispatch him quickly. He has a number of high-damage spells, but you can roll through most of them to get behind him, and then land a backstab. Sometimes the third sorcerer will join the fight, which is why we want to finish it quickly.
Next, continue following the stone path around and upwards to the chest on the other side of the room, which contains the Crystal Staff. Then drop down to the area below to find a chest containing a Rune Arc, head up the short staircase, and turn right to head out the doorway. There may be one more sorcerer, likely on your left. With him down, all that’s left is the boss of the area, Crystalian (Spear) and Crystalian (Staff).
Once you’ve defeated the boss(es), you can head up the stairs east into the elevator and ride it up — at the top, head outside and follow the stairs until you reach a ladder. Climb that, and you’ll end up on a platform with a chest that contains the Terra Magicus sorcery. With that grabbed, you can teleport back to the entrance to the dungeon, and then back out into the lakes!