give something under (the) threat of something
to give something only because one is threatened. He gave the money under threat of exposure. I think that is blackmail. You are asked to give your testimony under the threat of being jailed if you don't.
going to tell
and going to tattlea threat that one is going to report someone's misdeed to someone in authority. If you do that again, I'm going to tell! Sue just went to the teacher. She's going to tattle.
Never make a threat you cannot carry out.
Prov. You should not threaten to do something you cannot do; otherwise, people will not believe you are serious when you threaten. Bill: If you don't stop being rude to me, I'll have you fired! Jane: You're not my boss. Never make a threat you cannot carry out.