Ah, that one. It's about an extremely powerful reality warper that relentlessly tortured researcher James Martin Talloran in all forms and manners, ranging from physical beatdowns to repeatedly killing his family in various gruesome manners.
Nonstop. For millions of years.
And the amazing thing about it? He didn't even break.
A modified excerpt of his last dialogue with his tormentor before finally defeating SCP-3999:
"Finally, This is how it should be. The scientist interviewing the anomaly. I am the one in charge now. I have brought order.
It's staying this way.
Do not threaten me now. Without me, you wouldn't have been able to achieve any of this!
Even you cannot survive without order. You latched onto me, and still need me, a pathetic excuse for order, to exist. This is pathetic. You're pathetic.
You can't frighten me anymore. For the first million years of nonsensical containment procedures and tortures and dream logic, it was the worst pain I had ever felt, but I survived. For the second million years of nonsensical containment procedures, it was still the hardest thing I had ever done, but I survived. By the third million years, I was growing numb. There's only so many times you can watch anything before you grow numb. But you know what, you motherfucker? I survived. Which is more than you can claim, you dumb brute, because you never lived at all.
If I end you, things will return to normal. I refuse to believe there's more of this. Of you having the O5 council abuse my mother with a…a…oh I dunno, the corpse of Jack Nicholson made of Fritos. Or something equally stupid. I refuse to believe the only thing left in the entire multiverse is your stupidity.
So who are you, exactly? Ask yourself that. Who are you before a human who is ready to fight. You're nothing but the primordial ooze. And I am ready to fight. I am numb to your bullshit, because here's the thing about horror and weirdness: the more you reveal of it, the less effect it has. I am sick of your horror. I am sick of you.
I'd say see you in hell, but we're already there."
Talloran's a damn chad.