The Hall of Hadar

From the introduction:

Most people experience a fear of the dark at some point in their lives. The uncertainty and unknown fill us with dread. There is often a fear of something materialising in that void, trying to draw you into it. What happens when those fears become a reality? That’s what this encounter is all about. The players enter a room where they see nothing and must navigate their way to the other side to escape. If only it were that easy though.