Answer:
Terrorizing
Step-by-step explanation:
Even though the father´s plan was simply to persuade the child not to take long baths (probably to save water or money) what he really accomplished was terrorizing the kid. By describing the monster so vividly the kid (who will obviously beleive in his father words) now thinks the actual monster exists and will hurt him any time in the bath.
If you look up in the internet terrorizing you will find something like this: "to make someone feel very frightened by threatening to kill or hurt them" (Cambridge dictionary) and even thought the father was never actually treathing on hurting the kid, he put in his mind the idea of a monster that will, and that´s why I think this example could be categorized as terrorizing.