A fearful thing is a thing that causes people to be afraid. In this case, people will fear God when they see what he does. AT: "what I do for you will cause people to fear me" (See: [[rc://en/ta/man/translate/figs-idiom]])
# I am doing with you
Here "you" refers to Moses and the people of Israel. (See: [[rc://en/ta/man/translate/figs-you]])