Achilles

Is it possible the masked woman is Shalash? Seems like it would fit nicely with Shalash destroying all her own statues. And if the heralds are pursuing the opposite of their paths training/aiding thieves and Ghostbloods seems about right for the herald of creativity and honesty. Weaving darkness as it were.

Pretty sure I recall reading there was a splinter or sliver. I could be mistaken. Clearly, something is having an effect on this world.

What do the 3 rules have in common? I think that might be a key to the shard's identity. The first thing that comes to mind is oxygen. Fire uses it, blood transports it, and running causes rapid inhalation. Silver doesn't rust. It tarnishes becoming covered in a black film. Oxygen. Oxidation. Chemical reactions. What that has to do with the Cosmere I don't know.