Originally I believe these were our first glimpse at the brazen beasts, but it’s been suggested—and I think it’s right—that these are in fact the Sons of the Harpy—their masks are all the same, and seem like a variation on the Harpy’s face depicted elsewhere. In the novels, the Sons of the Harpy are simply anonymous killers—they can be anyone in Meereen, which is why their reign of terror is so terrifying—but by wearing masks they kind of deliberately pick themselves out.
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