There could be any number of reasons for the Basilisk to slow down.
It could have grown tired after chasing after Zach and the others. It was dead just recently, after all.
It could have run into an obstacle like a big tree or a mountain, which forced to to slow down and take another route.
It could have encountered a monster powerful enough to threaten it instead of stirring its hunger.
Maybe the principal had come to save his precious students.
Out of all of those options, the last one was the least likely, but not by much.
The most likely reason for the Basilisk to slow down could be only one thing. It had arrived at the Hole and was preparing to descend.
Zach, however, had not arrived at his intended destination. He could feel his stomach rise as the snake's upper body soared upwards, only to take a sudden turn and plunge downward again.
