A "threatening note" is a message or letter that contains harmful or dangerous intentions towards someone, which can make the person feel scared or worried about their safety. Full definition
When threatening notes start to appear in her purse and gruesome dolls turn up in her office chair, she begins to realize that the threat is real. (bookpage.com)
One angry father manipulated teenagers to leave anonymous threatening notes at the ex-wife's home. (thelizlibrary.org)
But Kevin later receives a more threatening note that reads: «This is your last warning to replace Cheryl. (businessinsider.com)