The Shadow Realm

The Shadow Realm is the name given to an anomalous location people appear to be warped to when playing anomalous levels that are marked as non-canon, dubitations, joke pages, ztar articles, or flat-out deleted on the MIPS Hole Wiki while they are inside of the level. The original level will be removed entirely, including the entry, making it impossible to enter the Shadow Realm again unless another level meets the requirements. The Shadow Realm is a large, black void with several strange assets and pieces of geometry floating around that allow for the player to navigate this area, similarly to Bowser in the Broken Realm and Bowser in the Randomized Realm. Every time the Shadow Realm is entered, it appears to grow in size, potentially due to the nature of the method of accessing it.

Description
The Shadow Realm is a large black void with several pieces of floating geometry and objects resembling assets from the levels used to enter the Shadow Realm. The level is believed to be around three times the size of a standard Super Mario 64 level, likely due to it's strange growth every time a level meets the requirements to warp players to the Shadow Realm. The Shadow Realm does not appear to contain any power stars, coins, enemies, or any object that can be interacted with excluding geometry. There have been reports of portals out of the Shadow Realm to an unknown area if the player reaches a certain point, though these reports are unconfirmed.

Reports
These reports are in the order of which they occurred, meaning the instances of the Shadow Realm will be inconsistent due to the level's growing nature.

Report 1
This report is the first ever recorded instance of the Shadow Realm, though was simply brushed off before more reports of the Shadow Realm began to appear.

"Okay. I found some pink pipe in the Castle Courtyard, and jumped in it because I had no clue what it was. Suddenly, I fell into some weird black void made of random platforms made of clouds, some weird volcanic dirt, a rainbow pathway, and some weird castle-looking thing? I jumped around for a bit and explored the relatively small area I could, before I eventually fell off the map and ended up in the Castle Courtyard again. Someone jokingly called it the Shadow Realm, and it's where "dumb levels go", but I don't know. The pipe was gone, by the way."

It is worth noting that if the "weird castle-looking thing" is indeed Big Boo's Castle and not some other level such as Unused Level 1, it was marked as non-canon some time after this report. It is unknown what caused this anomaly. It is also believed the clouds are derived from Cloudy, Cloudy Dream, and the volcanic dirt originates from Fire Bomb River, as both were pages on the Wiki deleted a long time ago.

Report 2
"Okay, I found some elevator in the castle, right? I entered it, and it led me up to some area called the sixth floor. I was kinda skeptical I was playing a real copy at this point, but looked around and found some weird painting of a city. I jumped in it and boom, I fell into some place that resembled some weird void full of random objects like a building, an image of a cat, some orange, green and white trees, and a ton of other weird miscellaneous objects. I went to the pause menu, and the name was just gibberish. So, I was roaming around for a bit, and then I found some weird building made of black bricks. I entered the door on it, and then I got warped back to the castle. No elevator, probably no sixth floor, nothing. Odd."

This report appears to indicate the Shadow Realm gained assets from the sixth floor and it's City in the Night Sky level, the Cat Room, the original version of Seasonal Sanctuary before a new one was discovered, 15 CRSE, and likely other levels that are not indicated by the report. It is not known if the Seasonal Sanctuary remnants still remain now that a new one has been established, due to the difficulties in locating the Shadow Realm.

Trivia

 * Despite falling under the conditions to be "warped" to the Shadow Realm, the levels associated with Among Us Manifestations appear to be resistant against the Shadow Realm, implying some property unique to those levels that does not apply to any other levels that meet the conditions to theoretically warp to the Shadow Realm after the conditions to do so are met.