In this part of the story, Macbeth runs into the three witches who tell a prophecy to both Macbeth and Banquo. This part fits because the witches in their prophecy were ambiguous, which terrified both men at how they knew such things, especially as one came true right after the fact.
During this scene where Lady Macbeth makes quite the shocking monologue after receiving her husband's letter, her true colours are revealed. She comes off as borderline, if not completely, insane as she asks the world to "unsex" her and to come to her woman's breasts and "take my milk for gall." So in this scene Lady Macbeth is terrifying because of her shocking words.
When Hecate confronted the three witches about not letting her in on the fun of messing with Macbeth regarding the prophecy. Because of this interaction, Hecate presents herself as an intimidating and almost sadistic being since she wants to help them mess with Macbeth's head. All in all, this brief lecture, we see her as quite the force to be reckoned with.