。
He trembled at the horrifying sight.
Literal
Horrifying [attributive-な] sight [at-に] he [topic-は] trembled.
戦慄する ('to tremble, to shudder') is a formal/literary verb for shuddering with fear or horror. 光景 ('sight, scene') is used for vivid scenes one witnesses. に marks the trigger/cause of the emotional reaction. A terse dramatic sentence.