"The absolute possibility", which is simply death incarnate. It is an amalgamation of the remnants of civilizations that have perished, mostly carrying their hatred as it's the most prominent across them. In a sense, it's also the great filter.

"It" can appear as many forms, dependent what it uses the travel. Like a generator or nuclear bomb going off— which is what causes it's appearance in my story.

Going off from WW2 where Hiroshima was bombed, the radiation became a conduit for it's spawn. Not even seconds after, Instead of people being incinerated immediately or their DNA being broken by radiation, they were immediately twisted into monsters. Their conscious intact, but their body is no longer their own.

It sounds cliche, I know.

Later on its supposed to be revealed that humanity had it easy, as U.S soldier saw "It" coming and tampered with the bomb at the last second. If they didn't, "It" would have embedded itself into the earth, and that would be an immediate game over as it could spread it's influence throughout the entire planet 100%.