Final answer:
Fezzik's greatest fear is terror and the absolute effect of it on him.
Step-by-step explanation:
Fezzik's greatest fear is terror. He is consumed by dread and becomes increasingly anxious about the future and how it will affect him. He is particularly terrified by the idea of any incident, no matter how small, that could further unsettle his already agitated state of mind.
The fear he experiences is not of danger itself, but of the absolute effect of terror on him.