The Staircase in the Woods by Chuck Wendig
While on a camping trip, five high-schoolers bound by an oath to always protect one another discover something in the middle of the forest: a mysterious staircase to nowhere. One friend climbs up but does not come back down. Then the staircase disappears. Twenty years later it reappears, and the friends return to find the lost boy—and what lies beyond the staircase.
