Nothing

"'No! No! I don't ! Nothing!'"

- Scientist, before becoming a victim of the Nothing.
The Nothing is an immaterial and apparently non existent malevolent force that inhabits the Mesa Black research facility. Whether it appeared before or during the With invasion is unknown, however it can be inferred that it has a vulnerability to a substance known as "grave", beer and the government as it seemingly stopped attacking individuals after it was exposed to these attributes.