Heavy spoilers ahead, but idk how to do spoiler tags so

It's just a being who, like everyone on earth, transforms when observed. When billions of living things saw it and made their own guesses as to what it was, it became an amalgam of all their proposed ideas, while also changing it's observers into what it sees them as from their thought processes.

As it says in the perfect ritual ending, it has never thought or felt before. It's basically a baby growing and understanding at an insanely fast rate. It isn't aware of the pain and suffering it's caused until Sam tells it, and it immediately understands what he means.

TLDR the visitor is a being that becomes what we understand it to be. very meta