This area is where you are able to take revenge on Seath the Scaleless and contains numerous invisible paths that you must find by watching the falling crystal snow hit the invisible ground. It is accessed through The Duke's Archives.
You'll easily see the Crystal Cave from the inner garden of The Duke's Archives. You don't need to dispose of all the Crystal Golems on your way through the garden, as you can quite easily out-run them. Head straight for the opening and start making your way down the crystal walkways.
Inside the cave, after making it past the first Crystal Golem (again, if you run past him he will stop chasing you eventually) you will reach a point where the crystal walkway makes a Z shape, but don't follow this path just yet. Before you turn the first corner of the Z shape, on the left you will notice an opening with some snow falling and hitting off thin air - or so it would seem. The snow is actually hitting one of the Crystal Cave's invisible paths.
The invisible paths are completely transparant walkways that are dotted around the cave. You will need to traverse at least two of them to proceed deeper into the cave and they generally get harder to cross the deeper you go. While there are some that are unavoidable, there are also some that are non-essential but lead to secrets, such as this first one. The snow that you see falling is the signifier that such a walkway is present and it helps greatly in crossing them. As the snow hits the walkway you'll be able to determine where it is safe to walk. Prism Stones are also invaluable for crossing some of the later ones as they let you know if the area in front of you can be walked on and also then leave a coloured marker at the point where it dropped, creating a trail that you can easily follow should you need to go back across the same path again - it would be adviseable to save them for a secret invisible path you'll encounter later.
This first invisible path is in a straight line, and if you simply follow it forward you'll find Humanity on a dead body. After that, continue on down the visible path that you just deviated from and you will come across a Moonlight Butterfly perched on a ledge. There's a few of these dotted around the cave and it's highly advised to simply ignore them, as they will attack relentlessly if you attack or move close to them. Keep following the path around and you will see snow falling on your left at the end - signalling another invisible path. Before you go to cross it though, have a look out over the edge on the left as you approach the start of the invisible path and you should notice a Golden Crystal Golem waiting on a large crystal walkway. If you fought the one in Darkroot Basin or the one just outside the Crystal Cave in the garden of The Duke's Archives, then you'll know that these guys are very powerful - so be careful.
It's in your best interest to lure the Golden Golem over to you rather than attempting to fight it on the narrow walkway it's on, or on the invisible path that leads to that walkway. So, use arrows to grab its attention and it'll then start heading over to you so you can fight it on a larger area of solid ground. ( Tip: For an easy kill, wait at the edge close-by to where the invisible path ahead starts and keep firing arrows at it while it comes across the invisible path - there's a possibility that it'll attempt a jump attack at you from the path and fall to its death in the process). Once it's dead, head across the invisible path, this one goes in a straight line directly toward the large crystal walkway ahead, but again only walk where you can see the snow land, because you can easily fall off if you're not careful.
Once you make it across the invisible path you'll be on a large crystal walkway, but before you go to the other end of it there is another Golden Golem that you can reach from here. You do not need to engage this one (it will leave you alone as long as you leave it alone), however if you are feeling brave you can reach it by carefully walking off the other side of the walkway from where you got onto it. There is another invisible path here that leads down to another crystal walkway, and as you approach it you will be able to see the second Golden Golem standing at the end of it guarding a corpse with an item. The limited space for this fight makes it quite tough, so good luck. ( Tip: Using the same strategy in the tip above; if you wait on the invisible path close to where it meets the walkway and lure the Golem over with arrows, there is a chance that it'll also attempt a jump attack from the walkway to your position on the invisible path and fall to its death) Beware that this walkway is very easily to slip off of - one side is slightly steeper than the other, so avoid that side of the walkway to avoid falling. The body on the walkway holds a Blue Titanite Chunk. Head back up the invisible path that brought you down here and then make your way across the crystal walkway at the top until you reach solid land again - be careful not to slip off while making your way across.
There is a Moonlight Butterfly to your right at the top of a crystal ramp, and giving it a wide berth is advised. On the left of the platform (facing the butterfly) you will see falling snow, revealing another invisible path. Walk off the edge here (it's only a short drop) to land on it. Make a quick 180 degree turn to reveal a tunnel behind you and run inside to quickly kill the three Crystal Lizards in here before they disappear. ( NOTE: If they disappeared before you got them all, a quick quit-and-reload will make them reappear so you can get them.) Continue through to the other end of the tunnel to find another invisible path, but this one is not straight like the other ones - about two thirds of the way across there is a bend in the path. The key here is patience, only moving when you are certain - even if it means waiting a bit. ( Tip: With this invisible path, more than any of the others, Prism Stones are extremely helpful in determining where is safe to walk and they will also make the journey back much easier due to the trail they leave.) Once at the other end, you will reach another crystal beam where you will find a Blue Titanite Slab - so it's absolutely worth the journey.
Head back to the tunnel and out the other end to the invisible path you fell onto before taking the above detour. Again, do not worry about the Moonlight Butterfly on your left. This invisible path goes in a straight line, but you should still look out for the falling snow for assistance if you feel you need it. When you reach solid land again you'll see the cave ahead where you should proceed, but before you go in, take a left and look around for a corpse holding a Soul of a Hero - be warned though that when you pick it up you may incite the aggro of the nearby Moonlight Butterfly, so ensure you're quick to run into the cave once you pick it up.
Inside this cave you will come across some Man-Eater Shells. If you visited Ash Lake then you'll know that these things can take a lot of punishment and dish it out too, so don't let yourself get mobbed by several of them. Lure them one at a time and use pyromancy to dispose of them efficiently. As you enter their area also beware of the one hiding that's waiting in a little alcove to the left, you do not want to get attacked from behind. Alternatively, you can just run past them and head in to the large cave to trigger the Seath boss battle. If this is the first time you have been here then the clams will follow you into the boss arena as there's no fog gate, but Seath's attacks can kill them if they do. However, if you have died already fighting Seath in this arena then there will be a fog gate in front of the arena for all subsequent attempts and the clams will be unable to follow you through.
Boss Battle : Seath the Scaleless
NOTE: Once you defeat Seath, remember to go back and visit the room where you fought him in the The Duke's Archives again to get some valuable treasure and also face off against a hollowed Big Hat Logan , if you've followed his quest-line correctly. To this end, don't rest at the bonfire (you may light it though) that has appeared in Seath's arena and instead use the Homeward miracle or a Homeward Bone to quickly go back to the Archives.
With Seath's Lord Soul collected, use a bonfire to warp to the Chaos Servant bonfire in Quelaag's Domain to continue on to the next area - the Demon Ruins.