Okay so she didn't join Autumn for the Fear, but for the knowledge, their rumored sorcery. She needs it. Desperately. Making friends and allies doesn't hurt either.

But sometimes causing a bit of fear was fun. And Whovians made it so fraking easy.

Well easy in a way that inducing the fear was simple. But the part where Mhairi could easily do that needed a bit of work. Which is why she had plans.

Plan A)
Step One. Identify a Whovian.
Step Two. Follow them home.
Step Three. Discover if they have statues, figurines,gnomes, etc. in their yard and/or garden. (If they have none put some there.)
Step Four. Move them slightly every so often.
Step Five. Cackle.

Plan B)
Step One. Identify a Whovian.
Step Two. If you are unable to follow them home, find them in their place of business, their common areas of recreation, (works best in a library.)
Step Three. Cut the power. (Or just turn off the lights.)
Step Four. Say "Hey, who turned off the lights?"
Step Five. Cackle.

Plan C)
Step One. Identify a Whovian.
Step Two. Sneak up on them while they are sleeping.
Step Three. Draw lost of tally marks on their hands and arms.
Step Four. Place the marker in their hand.
Step Five Cackle.

Plan D)
Step One. Identify a Whovian.
Step Two. Look at them with confusion.
Step Three. Wait for them to ask you about it.
Step Four. Tell them they have something on their backs.
Step Five. Cackle.

So it was a bit childish, and the initial idea might have sparked from her fiery flame of ill luck and trickery, but she's been so unsure of herself and sad for so long that maybe it was time to get back on her horse and...try for something else even if it amounted to childish pranks that invoked small sparks of fear then peels of relieved laughter, but maybe it's just what she needs.