You decide the fox just made a big mistake, since you're a nine tailed fox you should have his magical powers now, and you're going to curse him in as retribution.
Still, you can't say you've cursed someone before, so this can be challenging. You probably better pick something very simple so you make sure you can get it right on the first try. The fox probably won't give you a second chance.
You take a deep breath, feel all nine of your tails spread out and you make your choice.