A tormentor is someone who terrorizes another person, deliberately making them suffer. If your sister keeps tickling you after you desperately beg her not to, feel free to call her your tormentor.
A tormentor is a person who torments, and both words are strongly related to torture — they share a root that means "to twist." A tormentor doesn't torture once, though, but inflicts pain over a long period of time. While tormentor is often used in a light-hearted way, as when you say your devilish new kitten is your Great Dane's tormentor, it can also be used with reference to serious situations.