I think there's already been a topic similar to this, but I feel mine is different enough to be it's own thread. Basically I think it's a pity so few people ever see the Unknown Passage, given how its atmosphere & design one of the more memorable levels in the game, so here's an idea for a similar level (known as "The Breach") that would appear in the Clockworks.

About the Level:
The Breach would appear at Depth 28 of a certain gate (it moves between gates like the bosses do), but would have the Icon & Name of a regular floor so that you don't know its true nature until you actually visit it (to make it a bit more unexpected)

While in The Breach, your minimap does not work, similar to the Unknown Passage. The background starts off similar to that of the regular Clockworks, but gradually gets clouded over by darkness/swarm over time (near the end it begins to cloud the level itself too). About halfway through there is a small tremor and the sound of something large moving, after which said tremors become a regular occurrence.

Uses Whitespace Ambient and Whitespace Swarm as its Level & Combat tracks respectively.

About the Level (alt.):
Alternatively, instead of being hidden, the Breach would be visible from the gate map (it's icon would be a swarm-coloured gear on a black background) but the Lift down to Depth 28 would only show a question mark (like the Graveyard/Treasure Vault depths), which would let people know where it is but not if the lift leads there now or not.
.
.
Level Design:
The Breach starts out like a regular Deconstruction Zone (or Clockworks Passage) with regular enemies, but as you progress further through the level they rapidly begin to be phased out in favor of Swarm Enemies. At first only the first 25% of the level is accessible, with the rest being covered in a "Super-seed" that does massive damage to you if you walk in it. For every enemy that dies the Super-seed retreats back a tiny bit, sometimes leaving behind regular Swarm seeds in its wake. As more enemies die more and more of the level becomes acessible, until finally the Knights can progress to end, which has an arena where they encounter:
.
.
The Miniboss:
At the end of the level you fight a Miniboss known as the "Swarm Shard", a Battlepod-sized creature with 6 legs, two long gangly arms and a large Swarm Turret for an eye. It has mild resistances to all damage types & statuses (i.e you can't Shiver him), isn't a part of any family and inflicts massive Normal damage on his attacks.

The Swarm Shard uses Boss 2 (the original version) as its theme tune.

Phase 1:
For the first Phase the Swarm Shard is holding on to the edge of the arena with his legs while he attacks the Knights with his Arms & Eye. He is immobile in this phase & possesses the following attacks:

Attack 1: A 2-hit combo where the Shard reaches out and punches the ground with one arm, sending out a shockwave, before performing a wide sweeping attack with his other arm. Both hits have a lot of reach owing to the Shards long arms.

Attack 2: Fires out a wave of 5 bullets followed by a wave of 3 bullets from his eye. The bullets are identical to those of the Swarm Turret.

Attack 3: Fires out a massive bullet that tries to home in on a Knight for about 5 seconds before exploding, leaving behind a miniature Swarm Seed that lasts for a few seconds. Just before the bullet explodes it stops moving, and can be shield-bashed in the split-second between stop and explosion. If it collides with the Swarm Shard the bullet will damage him when it explodes, as well as staggering him for a split second.

Phase 2:
After it has taken enough damage the Swarm Shard climbs up onto the arena. It is no longer immobile and in addition to its Phase 1 moves it now has 2 new attacks:

Attack 4: Both the Shard's arms morph into tentacles and tunnel into the ground, whereupon they burrow around striking people similar to Big Iron's pipes. This lasts for 15 seconds and the Shard cannot move while doing it (but takes less damage while doing this attack to make up for this) but can still use attacks 2,3 & 5.

Attack 5: The Shard extends it's neck so that its eye is now on level with the Knights. After a few seconds of charging the Eye emits a cone of light that Damages and Curses anyone caught in its light for around 5-10 seconds. Attacking the eye will do full damage to the Shard as opposed to the reduced damage its body takes.
.
.
Reward:
Upon defeat the Swarm Shard staggers to the edge of the arena and falls off the level. He drops a "Swarm Prizebox" for each Knight which, after being collected causes the barrier blocking the Knights from progressing to disappear and dozens of Swarm monsters begin to spawn in. The Knights must flee to the end of the level whereupon a gate closes behind them. The end room contains the lift down to the Core camp along with dozens of Treasure boxes (to make up for the fact that Swarm enemies drop nothing)

The Swarm Prizebox, when opened, contains one of the following, from most common to least common:

- Dark* Costumes & Accessories: Dark blue versions of regular costumes & accessories (data for them is already in the game.)
- Vein Eyes: Similar to the Normal eyes, but have Swarm-seed like veins coming out of them.
- Swarm Eyes: Changes the Knight's eyes to the Swarm icon
- Swarm Colour: Changes the Knight's Eyes, Accessories & Name to the dusky red associated with the Swarm, while the "personal colour patches" on their Armour & Helmets is changed to black (Example)
- Swarm Aura: An aura of Black squares similar to the effects in the Unknown Passage. The knight also leaves a slight trail of "Swarm-Seed" behind them as they move.

*Please note that the "Dark" style (which is dark blue) is NOT the same as the "Shadow" one (which is black with red glowing parts), as these pictures show:

Knight with Dark Fur costume & Dark Wings
Knight with Snarby set (the only "Shadow" style armour in the game to my knowledge) & Shadow Wings