Stone Cells
Items in this room:
Gold Coins: 3 | Keys: 7 | Potions: 5 | Gems: 1
Entrance Area
You're definitely going to want to make use of the Save Point here. There's a crumbled hallway to the left that has nothing of note in the Quest version, but has a Gold Coin pot in the PCVR version. There's some fun stuff off to the right, but for safety reasons we're glossing over them for now. From the Save Point, turn right and move to the T-intersection. Turn left and pause, then just start shooting at the next intersection until you start hitting an invisible Evolved Crawler that walks into "view." Keep hitting it, and back up a bit to keep out of reach until it falls.
Now that it's safe, smash open the pot near the pedestal in front of the relief for some Gold Coins (Quest version). Odin Vision from that pedestal will show a red brick on the left of the hall leading back to the Save Point. Pressing this brick will reveal a Small Key in the pedestal to Odin's left.
Head back to the intersection and shoot the wall to the right to reveal a Runic Panel. Shoot it again to get rid of the nearby red barrier. Turn back to the hall and head toward the next T-intersection. There's a blue barrier on the left, but more importantly as you approach the intersection you'll be charged by two Spiders. Take them out and then head to the intersection and turn right. Instead of dashing toward the Save Point, start shooting your Wand down the hallway to defeat an Evolved Crawler approaching from the other side. Once the way is clear, head down to the Save Point. Use Odin Vision to reveal a hidden chest, which can be opened to find a Large Key.
On the ceiling above the save is a Runic Plate. Shoot this to dispel the blue barrier from where you came from, then shoot the Runic Plate that was behind it to dispel the barrier currently beside you. Once that's down, a Big Mouth will swoop around the corner and attack. Take it out, then drop both barriers again and continue on. Round the corner and walk until you see a door on the left. Unlock it with the key you just picked up and head down the stairs on the other side.
Break the small green pot on the left by the pedestal for some Gold Coins, then summon Odin to it and enter Odin Vision. There are skull and crossbone marks scattered through the room, so keep note of their locations to avoid triggering those pressure plates. Also, you can spot a hidden chest to the right that contains a Blue Potion.
Dash across the room between the pressure plates. There's a barrel behind on of the pipes containing more Gold Coins. Unlock the chest here from the side to safely retrieve a Red Potion. Now approach the lever in the center of the room from behind. Push it forward, then Dash past it and run up the stairs a bit. Turn around and fend off the two Crawlers that were just released.
Hard Mode
In Hard/Ironman mode, there is a third Crawler.
At the top of the stairs, the gate that was previously blocking the way has now been raised. Head down the hall toward the alluring blue light of a Save Point. As you approach an intersection, a pair of Crawlers will charge out of the side hall on the right. Once they are dispatched, turn down the hall they came from. While dodging traps revealed by Odin Vision, follow this hall around a corner to a dead end. Shoot the wall at the end to reveal a Runic Plate, then hit it again to dispel a red barrier near the Save Point. Turn back and head to that new save spot, then turn right and head down the new hall.
Follow that hallway around some corners until you reach a lever. To the right of the lever is an illusory wall that you can dispel. The area beyond that wall is blocked by a gate, so pull down the lever to raise that gate.
Snowy Cavern
Step forth into the large snowy cavern. There's a Rune Panel off to the left that will release more goodies once you know the code. More importantly, however, there's an Evolved Crawler approaching you from the cavern's exit. Shoot in the general direction of the doorway until you find it and then quickly defeat it. Approach the exit, but take note of a raised snow-covered rock on the floor to the right. You'll be able to spot a 
rune. You can enter it into the panel now or just remember it for later, you're choice. Now head out of the cavern.
There's a red barrier to the right, so that way's no good. Turn left and follow the hallway until you hear the familiar shriek of a Crawler. One will charge you from around the corner ahead, but there's a second one that appears in the snowy cavern. The safest best is to turn around as soon as you hear the shriek and make you way back to the cavern. Dash past the Crawler approaching from the cavern, then turn and back pedal a bit more while firing upon both enemies.
Hard Mode
In Hard/Ironman mode, two Crawlers will approach from within the hallway. Safest bet is still to follow the Normal strategy.
Once the way is clear, continue down that hallway. Follow it until you reach a Save Point by a door. The hall continues past the save, but there's nothing to collect down there. Fire your Wand into the darkness to reveal a hidden Runic Plate. Hit it again to dispel that red barrier we just passed. Turn around and head back to where it was to continue on. Just past the former barrier, you can spot a hanging tapestry with an 
rune stitched into it. Keep this in mind for the Rune Panel and continue around the corner to the right.
There's a blue barrier at the end of this hall blocking progress. Use Odin Vision here to spot a nearby illusory wall, which you should summarily dispel. This reveals an alcove lit by a blue torch. There's a chest in here, but if you've been following along you'll be fresh out of keys. Keep this in mind for later. For now, just hit the Runic Plate to get rid of the blue barrier, then Dash through the arch it protected before it turns back on. Follow the hall to the left and pause just before you reach the next corner.
The hall once more turns to the left, but this is a magic missile hallway. So bring your Shield up before stepping out. At the end of the hall, turn to the right but keep your Shield up since this is another magic missile hallway. Reach the end of this one and once more turn to the right into a safe hallway.
I've seen videos where there's an Evolved Crawler in here, but that seems to have been patched out in the years since those were recorded. In the current release, there are no enemies here to worry about. You can use Odin Vision here to spot a 
rune in the alcove near the skeleton. Break the nearby barrel with some fire magic to find a Large Key. Make sure you don't break the barrel with ice, as this can cause the key to not spawn. Pull the lever to raise the nearby gate, which leads you to a spot near the beginning of the first magic missile hallway. You could put up your Shield and back down the hall to the safe portion. But it's much safer to creep to the edge of the intersection and do a well-aimed Dash directly to the safe hall.
However you get back on track, the goal is to head back to the Save Point on this side of the snowy cavern by the door. On the way, you can swing into the cavern and approach the Rune Panel. At this point we've found the , , and runes. I honestly have no idea where you're supposed to spot the fourth rune, but through process of elimination it turns out to be 
. Entering all four will reveal a Small Key. Heading back to the chest we found earlier will net you the final set of Gems.
With that side quest done, hit up the Save Point by the large door. I do recommend saving here. Unlock the door, then immediately pull up your Shield.
Underground Lake
Just on the other side of the door is a Poison Floater, so make sure you use the doorway to your advantage to avoid its spit. Roaming around in the room behind it is a Big Mouth, which will be continuously firing at you. Some of the fireballs can shoot past the Floater and hit you, hence the Shield. Take out the Floater, and take a step out onto the dock while taking shots at the Big Mouth. At the far end of the underground lake is another Poison Floater. Just concentrate on the spit balls until you take out the Big Mouth, then turn your full attention to the Floater until it's dispatched.
Odin Vision will reveal a hidden chest in a giant ventilation pipe on the other side of the lake. But ignore that for a moment and turn left. About halfway down the left hand wall is a barred window. Freeze the lake, then Dash over to that sealed window arch and pick up a Small Key nestled within the bars. Dash back to the dock before the lake thaws out. Freeze it again and this time dash into the enormous pipe opening and zap the hidden chest. Pluck the Large Key from the air, and keep holding it in your hand. Freeze the lake once more and Dash over to the door and unlock it.
Past the door is a Save Point on the left if you decide you need it. It certainly comes in handy for the next portion. Follow the hallway around a couple corners, then summon Odin to the pedestal that is past the snow. Hang out in Odin Vision until you see the fearsome snout of an Evolved Crawler appear from around the corner. You'll have just enough time, if you're daring, to rush forward and get off a shot or two before it can turn the corner into the portion of the hall you're currently standing it. The freezing method is great here if you're playing a version where it works, but otherwise just keep your distance and fire away until it falls.
Move down the hall the crawler came from (watch out for the Spider!) follow the twists and turns until you turn a corner to the left and see a gate further down. Spin around and face the way you came. At the end of the hall in front of you is an illusory wall. Behind the wall is an Evolved Crawler, so just start blasting in the direction of the wall to get enough hits on the enemy that it won't be able to charge you. Head into the alcove the wall was hiding and pull the lever to raise the gate you just saw. Move through the newly opened gate to reach a dead end with another lever. Pull this one as well and you'll hear the gate by the snowy hallway open up. Face away from the lever and take a couple steps forward past the torches, then dispatch an illusory wall to your left. There's a red barrier in play here, so head back toward that hallway with the snow in it.
Head straight past all the snow to enter this new area. Not much to it, but there is a chest you can unlock for a Purple Potion and a hidden Runic Plate that can be used to dispel that pesky red barrier. Now return to where the barrier had been and head to the Save Point. Ease around the next corner to spot a blue magic missile circle. You're too far away to be spotted by it, but you will be within the detection range of a Crawler. Take it out before it can reach you, then pull up your Shield and make your way down the hall. Turn left at the end into another magic missile hall (avoid the center of the hall, as there's a pressure plate just as you turn the corner). Turn right at the end of this hall to enter a safe area.
Follow down a couple corners until you reach a T-intersection. There's a deadly pressure plate just in front of the pedestal to avoid. Move beside it, then face the way you came and look up. Just above the "doorway" you came through is a Runic Plate that dispels the red barrier (the PCVR version just doesn't have the red barrier). Fire down that section of hall to hit the Runic Plate that turns off the blue barrier. For a wonder, you can move past the disabled barrier without getting attacked by anything.
To the right just before the next blue barrier is an illusory wall. Blast that to reveal another alcove with a Runic Plate and lit by a blue torch. Before hitting the Plate, however, there's some goodies to grab. The recess on the left holds a Purple Potion, and the one on the right has a Red Potion and the final Large Key. Once you have everything, zap the Runic Plate and head through the barrier.
Dash over to the Save Point to avoid the magic missiles that will detect you as you pass the barrier. When you're prepared, pull up your Shield and travel down the hall. Move to the left as you reach the end to enter a safe zone blocked by a gate. Face the intersection you just came from and zap the wall across the way to dispel an illusory wall. Dash across and pull the lever, then Dash back and head under the open gate.
Halfway down this hall is an intersection with a Healing Fountain to the left and a Message Sphere to the right. Use as many of those as you wish, then continue on to the end of hall and unlock the door to the final room.
