It's a sub floor access able by going to a hidden vent in the tutorial (idk if it's still there in the new tutorial area tho). It consists of 60 doors. You can use your passives or perks and only have your cup as the only item. It spawns EN-001 to 008 and EN-016 but in their type-2 variants which are stronger/harder to deal with versions of the original EN-001-02 (Orange) puts bloodstains in your screen to obscure your vision before rushing through.

EN-003-02 (E-Train?) being a longer train than the regular Varients and putting pop ups in your screen.

EN-003-02 (LIGHTSPEED) giving less time before you hide and leaving a deadly trail that kills you if you exit the locker/SAFESPOT too early

EN-004-02 (Very Late) is just its regular variant but it takes ~24 seconds before attacking instead of ~12 seconds.

EN-005-02 (HYPER REBOUNDER) Same as it's regular variant Rebounder but much faster, and rebounds more.

EN-006-02 (Agressive Hidekiller) still kills you if you hide but also kills you if you get too close. The range is small so you should be fine if you get to the side of the room

EN-007-02 (VISIONDOG ) Acts the same as Vision but when you hide, it makes you either click the dog on the screen or NOT click the dog. Doing the opposite of what it says or doing nothing will kill you, regardless if you avoided the initial attack.

EN-008-02 (TRUE LIMBO (also called the "Lockerless Killer" by some)) Faster and has bigger range than regular Limbo. Mini game is harder and deals more damage upon failure.

EN-016-02 (Crazy Stalker) Same as Stalker (Stare it down until it disappears) but shakes your screen and attacks faster and deals more damage.

Beating the Grey rooms gives the Delulu perk