Need help remembering this short story/the book it came from
This is what I remember - the description is choppy, because I only remember bits and pieces. I read this quite some time ago.
A boy puts sock on cat's head and in it's scared/confused state the cat goes under his bed. Then the cat disappears. Eventually the cat, now evil (though I'm not 100% sure if it was evil or not) returns, talks to the boy and the boy finds out there there is a "portal" under his bed to the land of nightmares. The boy somehow gets sucked into the portal and has to walk, in his bare feet, towards the guy who runs the land of nightmares. The boy is scared. Long story short, the boy, who was a bad kid apparently, is now trapped in the land of nightmares and has to spend the rest of his life/eternity delivering nightmares to people.