Oh, thank Celestia you can hear this. Let me explain why you aren't hearing Pinkie at all before you go 'Mena, stop controlling our friend' on me, alright?
Pinkie gave me permission to take control cause I can a) Fight better than her and b) She's too overwhelmed with this place to be able to keep control of her sanity.
So, I volunteered. That's what happens when you have another version of you in your head.
So now that that's out of the way, I'm a bit concerned about the fact that I hear screams outside. Yeah, I know, the serial killer shouldn't be scared of something she's used to. Here's the thing: There are screams that I'm used to, and there's also ones that sound like some crazed beast is screaming continuously.
Not only that, but I'm hearing roars in the distance and what sounds like teleportation in the walls.
Celestia only knows what's happening out there, but I think it'd be best if I stayed hidden, yeah?
And to you, Midnight Moon or whatever your name was, they're not doing a very good job of Securing, Containing, or Protecting by the banging sounds and then a scream.
So somepony tell me what the buck is going on here, ok?
Wait, why is there black goop on the walls..? And what the buck- OH BUCK NO! I'M OUTTA HERE!
Shit. What you just encountered is SCP-106, nicknamed 'The Old Man'. He's a Keter-class SCP, among the most dangerous there is, and he's actively dangerous. He can... well, 'pass through' isn't entirely accurate. More like he's able to enter a pocket dimension by going through walls, floors, and ceilings, and exiting through them as well, leaving that black goop you saw at the point of entry. DO NOT touch it, that stuff is highly corrosive, especially to living beings. It's been compared to stomach acid, and he's covered in the same stuff.
As for The Old Man himself, he's among the deadliest SCPs the Foundation has. He hunts people. He'll incapacitate his prey by damaging organs, muscle groups, or tendons, then pull them into his pocket dimension, where he 'plays' with his victims before finishing them off. He has full control of this dimension, and can alter time, space, and perception inside it. Frankly, if you get pulled into it, you're as good as dead already.
His ability to 'pass through' solid matter is what makes him so difficult to contain. When he breaks out of containment, standard protocol is to lure him back into his cell (after it's been repaired or replaced)... by injuring a human, ages 10-25, usually by breaking the femur, and transmitting the screams of pain over the sound systems. That usually attracts him to the lure after about 10 to 15 minutes. Unfortunately, from the sounds of things, they're not going to be able to set up the lure for quite some time.
Now, if it was just The Old Man, I'd advise you get to large, open, brightly lit room. The Old Man's adverse to bright lights, and you can easily tell where he's about to exit his pocket dimension by that black goop forming. However, that crazed screaming you mention? That raises two possibilities. A; it's SCP-096, 'The Shy Guy', in which case you're fine as long as you don't look at its face. B; it's SCP-682, 'Hard-To-Destroy Reptile'. That thing can't be killed. At most, you could damage it enough to temporarily incapacitate it. If it targets you, just run. And as much as you and Pinkie might not like it, if you encounter any humans, put them between you and 682. They're trained in subduing that creature long enough to re-contain it, and they, each and every one of them, are willing to give their lives to do so.