I Found a Hidden Door in My Basement…

I moved into this old house a week ago. The basement always gave me the creeps, but I chalked it up to bad lighting and my overactive imagination. Last night, while organizing boxes, I noticed something strange: a faint outline of a door in the wall.

It wasn’t in the floor plans.

Curiosity got the better of me, so I pried it open with a crowbar. Behind it was a narrow staircase leading down. At the bottom? A single chair facing the wall, and on the wall, there was a sentence scrawled in what looked like dried blood:

"If you’re reading this, it’s already behind you."

[What should I do?]