"Ghost Adventures" star Zak Bagans tells TMZ, the supernatural encounters began after he found a human skull during an investigation last month at an old mining town hotel -- and genius actually decided to bring the remains home with him.

Zak -- who built a display for the skull in the basement of his Las Vegas home -- tells us, he noticed a huge surge in paranormal activity almost instantly ... an angry, demonic-type presence that had never been in his house before.

On one night, Zak tells us, he was woken up by a slight pressure on his ankles -- what felt like hands grabbing him -- and all of a sudden, the force yanked him several inches down in his bed.

Days later, Zak says he had a girl over at his house -- and out of nowhere, she spilled head over heels down his stairs ... insisting someone ... or something ... had pushed her.

But that's just the tip of the iceberg -- and Zak tells us, he's so terrified ... he hired a local bishop to perform an exorcism at his house in the next two weeks