Here "terror" represents the things that will cause them to be terrified. Alternate translation: "I will send disasters that will terrify you" (See: [[rc://en/ta/man/jit/figs-metaphor]])
The phrase "in vain" means that they would get nothing from their work. Alternate translation: "You will plant your seeds, but you will not get anything from them" (See: [[rc://en/ta/man/jit/figs-idiom]])