AI Headquarters

AI Headquarters is an area in B3313.

Description
It takes place in a large, dark, creepy and rusty labyrinth, shrouded in darkness. This location hosts the Core and there are two versions of it: one with a seizure warning by the entrance and another with suspiciously convenient arrows on the walls that point to the goal. Both versions make Fast Travel unavailable.

After enough stumbling in the dark, the player will make it to the checkerboard floor room where the Core rests. The closer, the more nearby textures will become buggy and the message "DESTROY IT" will appear. If this happens in the "hard" version of the area, the player can smash the Core and the game will crash like it does in the AI Underground area.

However, if the player is in the "easy" version of the level, when they come very close to the Core, everything will descend into darkness, and the Core will disappear. But in the distance, the old familiar Textureless Mario is already rushing towards the player...

Sign text (in hard version)
WARNING! IF YOU ARE PRONE TO SEIZURES

THERE IS A CHANCE YOU MAY ENCOUNTER

AN UNFINISHED AREA WHEN TESTING, IF YOU SEE

ANY AREAS WITH AN EXPERIMENTAL LOGO, OR

A MARIO ICON WITH AN EXCLAIMATION POINT, PLEASE

EXIT LEVEL IF YOU ARE AT RISK, AND IMMEDIATELY

REPORT TO THE HEAD OF QA.

THANK YOU FOR YOUR COOPERATION!

Origins
It appears to be an original creation.

Sounds like the Haunted House theme from the Vanilla Game OST.

Plot meaning
Mario/Luigi seems to have snuck into the AI's lair, which (obviously) pissed him off as his shouldn't be able to find this place. That's why AI pulled that awful Core trick on the Mario Bros again.

Leading here

 * Personalization Chamber: With some probability, you'll either get sent to the Easy version, or the Hard version of the level.
 * Flesh Mountain: Entering the cave at the top of the waterfall of blood will always lead to the Hard version of this level.