Staircase In The Woods - Chuck Wendig

Five high school friends, bonded by an oath to protect each other no matter what.

On a camping trip in the middle of the forest, they find something extraordinary- a mysterious staircase to nowhere

One friend walks up - but never comes back down.

Now twenty years later, the staircase has reappeared, and the friends return to find the lost boy - and what lies beyond the staircase in the woods...