To access this level, you'll need to visit a few more areas in the other levels to complete a two-part puzzle. In Seven Portals, up on the puzzle structure you probably saw a couple levers in addition to the switches that opened the final door. Pull the levers if you didn't already: they open those two "barred" doors in the Guardian of Ice.
Accessing the secret level[edit]
Go to Guardian of Ice--the east-side portal is easiest. The two doors are just outside the dark room you first entered the level in, and lead to two side areas, referred to here as parts 1 and 2. Part 1 is the black door to your right; part 2 the rusty one to the left. Inside each side area is a minor puzzle which opens a portal to another area in the episode. Consider getting both side areas cleared out and opened up before entering either portal, and enemies won't wander around and get in the way. (Start with the part 1 side area. It's crammed with Afrits that, if you get them taken care of first thing, won't be flying everywhere and infesting the area.)
Side area to part 1[edit]
Inside here is a deep pit surrounded by a narrow ledge and a small back area beyond, and to the right is a lower area. Find and pull the switch in the lower area, and a tiny room across the pit will open. Jump--or better yet, fly--here, and pull the switch inside. A portal in the back area will open; you can see this through the small window in the room. Watch out for fireballs as you jump/fly across the pit.
Part 1[edit]
After entering the portal, ahead, a neat door made of several sections will open. Past here is a medium-sized room of rusty metal consisting mainly of a deep pit mostly surrounded by a narrow ledge. Follow the ledge to the left around the pit, and run past the wall made of square panels. This section will move out and shove you into the pit if you take too long. Beyond the pit, wait a second, and a narrow walkway in the pit will rise, leading to the pillar. A lever on the pillar will be uncovered. Pull the lever to complete this part. The pit will begin filling with lava, removing the falling hazard, and then a door at the far end of the room will open. Go through here and you'll see you've been in a new part of the Guardian of Fire. Return to Seven Portals, and if you haven't done part 2 yet, go back to Guardian of Ice.
Side area to part 2[edit]
Step slowly through the door: a gauntlet of projectiles in the short passage inside will activate. It's easy enough to circumvent with timing. Past here, to the left is a small frozen room with what looks like a throne of sorts. On the wall behind it is a poison-dart launcher that will activate in a bit, so be aware of it. Find and pull the switch in the room: the launcher activates, an ice column blocks the exit, and a couple enemies spawn. Kill the enemies, and the ice column will be removed, plus a portal opposite the passage will open.
Part 2[edit]
Get good weapons ready: when you enter the portal you'll end up in the middle of one of the nastiest fights in the game outside the finale, especially if you're playing a difficult skill level. You'll be in a small icy cave, under fire from a bunch of Wendigos. Stalactites and stalagmites get in the way. When you kill the Wendigos, the outer wall lowers, revealing a much larger cave and many more Wendigos. Basically, don't stop shooting while any of them are nearby, and have your Quartz Flasks selected ahead of time. Explore the cave when you're done, and you'll see a doorway leading to a small, taller section of the cave with a big window up on the far wall, and discover this is the icy area in the Guardian of Steel. In the main cave, look toward each side for a lift that intermittently raises and lowers. You'll see these are the sections of "clean" metal in those dimly-lit rooms at the bottoms of the diagonal stairs.
The clean-metal sections in the rooms at the other side of the level are also lifts that lead up to that big rusty-metal room opposite the level from the icy cave, and will now be active. Go there now: you can take either lift but the northeast (right-hand) one is easiest. Step carefully into the room: when you do, crushers all over the place, made of upper and lower parts, activate. Be extra mindful of the ones whose bottom parts move very slowly. The top parts move much faster and can catch you off guard. From the northeast lift, follow the right-hand wall and hit the switch you come to. You're done in here, but there's a Mystic Urn available inside that cluster of crushers near the northeast lift, plus there are additional items you'll find if you explore the area.
Go to the central room: the switch opened up the middle area. Inside, jump onto the octagonal platform where the Torch is. This completes this part's objective and gives you the message "Stones grind on the Seven Portals." In actuality, this is true only if you already completed part 1.
Before leaving, there are a couple more bonus areas now open. Take the lifts from the central room down to the lower rusty areas. In each, you'll see that the big clean-metal barrier has slid all the way to the side, opening the entire area. (The barrier moved aside when the wall in the icy cave lowered; you can see it occur on the automap. And you'll understand now why there was a deep slot on the right compared to the small one on the left.) Where the barrier formerly was, there's a doorway to the cage between the rooms, with items. When you enter the cage, a door at the other end opens, connecting back to just outside the portal exiting the level. Earlier, you probably saw the handles on the outside of this panel and thought it was a door of some sort. You see now that it is; only it opens from the inside. Remember to go down to the lower area on the other side of the level too.
Portal[edit]
In Seven Portals, after completing both the Fire and Steel parts, go to the closed-off area in the courtyard where the water runs into. These are the stones that grind open. Inside that low flooded alcove with the grating at the back is a hidden portal to the secret level, Bright Crucible.
WARNING: If you're playing the Mage, get the Wings of Wrath before entering. There's one part that can be problematic with the Mage and can leave you stranded and unable to exit the level. But it can be circumvented by flying. Also, you might consider saving here in an additional slot if you have concerns about becoming stuck.
Bright Crucible[edit]
Most of this level is a huge outdoor area with left/right symmetry. You'll be on a high plateau with two rows of tall pillars. That yellow object with concentric rings is a Disc of Repulsion: this shoves all of the enemies ahead of you back, and deals damage to them as well. These will become fairly common beginning in the next episode. To either side is a big area crammed with creatures, and a big lower muck-filled reservoir between them, below and ahead. Wings of Wrath will allow you to get around a lot easier. If you don't have them, the plateau won't be accessible until later once you jump down. Be on guard in the water and muck: in here is a new enemy, the Stalker. These usually stay submerged but are occasionally visible, and will jump up right next to you and attack. They frequently attack from behind, too. Later on you'll encounter ones that launch highly-destructive gasballs and will often pop up at a distance to fire at you. At the far end of the muck reservoir is a lower muck-filled channel. The level exit is under the outer wall down here, but won't be open now. Finally, there are two lifts leading up and out of the muck reservoir.
Heart of D'Sparil[edit]
First, go through the whole area and get the enemies cleared out. Then, go to the side of the level that was to your right when you first entered. Here (and on the other side) are two high platforms, one accessible by lift. Go up and inside: in here is a long meandering corridor with side rooms that will open, containing Centaurs. At the end is a room with fireballs being launched everywhere. In the middle of the room is a pedestal that will lower when you enter the room, that has what looks like a red diamond on top. This is the Heart of D'Sparil: run up to and collect it, and the fireballs will stop. (Try to skid to a stop on the pedestal, and the remaining fireballs will miss you.) The far exit leads back outside to the other high platform, where there's armor.
Pedestal[edit]
Now, go to the opposite side of the level, to the other two platforms. Ride the lift up and go inside here. This corridor is mostly a very long, straight section, and the side rooms release a huge crowd of Ettins. If you're playing the Mage, try running to the far end of the long section, then turn around and attack with your wand. You may have noticed the projectile the wand shoots pierces every enemy in the way until it hits rock. This corridor is probably the best and most satisfying area in the game to take advantage of this; you can clear out much of the enemy horde with relatively few shots. There are lots of items in the side rooms too.
Go to the end of the corridor and break the stained glass blocking the way. This often doesn't want to break easily; try striking or shooting at it point blank and from different angles. Inside here is another room full of fireballs. In the middle is a small ornate pedestal. Select the Heart of D'Sparil in your inventory, run up to the pedestal and activate/use the Heart, i.e., place it on the pedestal. The fireballs will stop.
Break the other window at the far side of the room, and go outside to the second platform. (The window won't break until the Heart of D'Sparil is placed.) This is where the Mage may have trouble: the window may not break at all with the Mage's weapons. If it refuses to cooperate, go back outside, fly to the second platform, and go inside and down to the other side of the window.
Upper area and exit[edit]
When you walked through the doorway past the second window, a very long stairway leading from the muck reservoir up to the plateau began to slowly take shape. Go up the stairs to the plateau, and to the outer wall at the back behind the pillars. A passage here has opened, leading up to a courtyard with several colonnades and a covered area with a pool inside. Clear out the enemies, then look inside the pool for what looks like a rook from a chess set. This is your first Icon of the Defender, which makes you invincible for a short time.
The level exit opened when you entered the pool. Go to the low area at the far end of the muck reservoir. A muck-filled chamber here is now open, with the exit portal inside, leading back to Seven Portals.