It'll come up soon so I might as well start it myself.

So, what is her cutie mark?

The movie doesn't show it, at least that I noticed. I'm unsure about supplementary material, though since I haven't noticed any art with it I assume that the prequels didn't show anything either. I know that there's a G3 pony that shares her colors and has a name similar to Tempest's real one. I'd be surprised if that pony didn't somewhat inspire Tempest's design.

So what do you all think it could be? Do you think we'll get an official answer?

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If you want my opinion, I think see doesn't have one at all, because of how she lost her horn, her friends, and left her good past behind. And as a result of going alone, turning evil, and making a new name for herself, she never earned her butt symbol (cutie mark), and she's possibly still a blank flank as an adult. But like I said: that's just my theory and opinion.

I think she has one because with all of her internal monologs in the book about how much of a freak she is, you'd think that being a blank flank would be mentioned. I would guess it to be a firework of some sort.