Crossing the first threshold
Having refused the call to adventure, the hero now faces the consequences of his inaction. The mentor cannot help him. The consequences must be faced by the hero alone. The hero has only one path he is able to take: Crossing the first threshold.
Entering the special world, the one he refused to acknowledge and accept, has now become his only choice. The ordinary world is left behind, not because of choice, but out of dire necessity. This crossing of the first threshold should be formidable obstacle. The hero must be both reluctant and frightened by the crossing, and the obstacle closes in around him like shrinking walls, until all that is left is the path into the unknown, less the hero be suffocated by the obstacle.
The crossing of the first threshold represents the first physical obstacle, and overcoming it entering the special world the only path forward. Death strokes its fist brush.