(Peel your eyes: the flapping cape reveals a blank flank)

What would this mean to us?

It means the artists/animators of the show didn't want to bother with designing a cutie mark for something that isn't meant to be shown.

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He might have been redeemable, if they'd only brought the Crusaders.

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Do you even realize how blasphemous you're being?

Sombra doesn't have a cutie mark because he's an umbrum, who apparently just don't get them. At least, that's the answer that Fiendship #1 gave, but the show seems to agree on this if Sombra didn't have a mark in this episode either.

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On the other hand, Flurry shattering the Crystal Heart only brought a blizzard down on their heads, not a mass of angry horse spirits.

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Was it ever directly stated that the continued maintenance of the Crystal Heart was necessary to keep the umbrums in prison? I don't remember, but I got the impression that locking them away only needed to be done once, and then the spell would hold itself. After all, Sombra never found it necessary to destroy the Heart itself to unseal them.

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I admit, my understanding of that storyline is piecemeal at best.

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Best go make some cards about it then.