Special Containment Procedures:
The human population is not to be told about SCP-9810, as they will be less likely to believe such. If anybody knows about SCP-9810, they are to be advised to not: attempt to find it. If someone approaches SCP-9810, they are to be taken away and put in containment.
Description
SCP-9810-a appears to be a one-hundred meter (100m) tall castle, found deep in the forests of [REDACTED]. A blue light source coming from planet [REDACTED] makes it so that the castle has a blue hue to it. It stays floating seven meters (7m) above the ground, with a staircase leading to it's front door. If a passerby gets close enough, the door will open by itself, and the passerby is able to enter the establishment.
Inside, SCP-9810-a is much bigger than what it looks like from the outside, and it resembles the inside of an abandoned factory and mansion. There are an excessive amount of large pipes and tubes in the ceiling, of which seem to be transferring something towards an unknown location. In the walls, there are large windows made of tempered glass that show the inside of a bloody tunnel. Walking further into the castle will trigger an anomaly to slide in the tunnel, designated SCP-9810-b. SCP-9810-b has been identified as an unicorn that has gone multiple modifications, such as it's skin being taken away and it's size, of which makes it fit in the tunnel. SCP-9810-b makes neighing noises towards passersby that trigger it's activation, in a manner that resembles distress.
After four kilometers (4km), the passerby will find themself in a wide and tall room. There are dirty pools, overgrown prehistoric and non-existent plants and an abundance of modified mythical, folkloric and/or legendary creatures, such as pegasi, faes, selkies and many others. If the passerby is spotted by some of these, they will attack them with magical powers seen in fairytales, and if the passerby is killed, it will be made into soup. However, some creatures, such as chimeras and centaurs, will try and help the passerby leave the castle.
There is one specific creature that stands out, which is an ancient melanistic chimera, referred to as "Cassandra" and designated SCP-9810-c. Cassandra is humanoid (anthropomorph) and is able to speak english and greek. It appears to be the leader of the place, settling down the other creatures whenever things get out of hand. It judges whether the passerby is to be eaten or not, and if not, it explains the history of how the place came to be, and what happened to everyone around it.
Creatures residing in SCP-9810-a are unable to come out of it, as they disintegrate whenever they do so. This is due to an observed anomalous effect that makes it so that everyone inside the castle ages much more slowly than in the outside world.
<ADDENDUM 1>
D-10389 is sent to investigate SCP-9810-a, after many tactics of making him believe. Transcript of bodycam.
D-10389: Aight... Can't believe they're making me do this... (D-10389 makes his way into the castle, using his flashlight to inspect the place.)
D-10389: Ohh... Smells crude in here. (D-10389 walks around, pointing the camera and flashlight to the windows.)
D-10389: Not even an actual window, huh?
SCP-9810-b slides in all of a sudden, startling D-10389.
D-10389: Oh, Jesus! (D-10389 stumbles back.)
SCP-9810-b: (Distressed neighing.)
D-10389: ...Well, this just ain't friendship and magic. (D-10389 inspects the unicorn's appearance.)
D-10389: Sorry buddy, I can't do anything to help ya.
SCP-9810-b: (Louder distressed neighing.)
D-10389: Stop that...
D-10389 kept walking.
(Timeskip)
D-10389: God, does this ever end? (D-10389 drinks a bottle of water.)
Suddenly, he finds himself in the big room.
D-10389: ...Huh? (D-10389 looks around, seeing all the creatures laying and flying around.)
D-10389: ...Goodness me... (D-10389 tries to touch a fairy, but it throws some powder in his hand, burning it.)
D-10389: Ow! Fuckin'...
(?): Shush.
D-10389: Wha... (D-10389 sees a nymph in one of the dirty pools.)
D-10389: ...Goodness me... (D-10389 tries to approach her, but gets headbutted by a headless mule, which knocks him down.)
D-10389: Shit! Get off of me! (D-10389 gets stomped by the headless mule on top of him, and some gnomes steal his bag. The nymph pulls his head down the water, trying to make him drown.)
A figure comes in the room, it's goat foot clicking against the floor loudly, causing the whole room to stop. D-10389 is able to jerk his head back up, and see Cassandra standing there, in it's old robes.
Cassandra: ...Give this man his items back. (It orders, and the gnomes do so.)
D-10389: ...Wha... (The headless mule trots away from him and the nymph disappears into the waters. D-10389 looks up and Cassandra, who's looking down at him, judgingly.)
Cassandra: ...Come with me, sir. (Cassandra says as it walks away.)
D-10389: ...Okay... (D-10389 follows Cassandra, picking up his bag.)
D-10389 find himself in an old king room, with three thrones in the front. Cassandra stands in the front of a large and real window, looking out as it drinks a cup of wine.
Cassandra: ...I see you are 'recording' this. (Cassandra looks at D-10389)
D-10389: ...Look, I swear, they made me record this, please... (D-10389 says, scared.)
Cassandra: ...Are you still recording?
D-10389: I... Have no control over it... Please...
Cassandra: ...A long, long time ago, when the nymphs adapted to land and the centaurs adapted to the legs we see today, humans, as a concept,, were born. They where much more adapted than us, in intelligence, strength and durability. Mazurite, the goddess of mythical creation, helped them be the strongest being that she's ever made... (Cassandra pauses, taking a sip of it's wine.)
Cassandra: ...They helped us, until we were suddenly discarded and removed from history. We were forgotten, and nobody believed in us anymore. We were forgotten...
(...)
Cassandra: Mazurite didn't like that, so she took her own revenge. She opened up a large lake of water that would make a wave as big as the existing castles from before, but only made one castle resistant to the large wave, one that would float off the ground. She ordered us, her previous creations, to go up to the castle. But something went so wrong, and half of the humans and creatures were wiped away. Mazurite felt embarrassed at her mistake, crying her tears out. She decided to depart away from the earth, as far as she could, and became a planet. And now, humans that see her or believe in her will become modified, as her anger is prevalent to this day.
D-10389: ...Why is this place so deteriorated?
Cassandra: It has been here since medieval times... And ever since she left, we couldn't keep up with ourselves without her. The unicorns became inflated and humongous, the fairies stopped healing us and everyone else just became depressed.
(...)
D-10389: ..Uh, how do I... Leave?
Cassandra: It's the door to the left...
(...)
Cassandra: But, if you tell your 'friends' anything, please inform them that Mazurite was not an evil woman.
D-10389 ...Okay.
D-10389 walks to the door, and gets teleported to a gas station restroom. He left unharmed, with the exception of a few bruises and cuts.