Shosh

Who knows why, but as to the what... First thought is that clearly whatever it was didn't actually accomplish anything - Cultivation isn't even wounded, to our knowledge. Second thought is that a Dawnshard (or Dawnshard derived item) is one of the few things able to hurt a Shard (aside from another Shard and, maybe, Nightblood... who wasn't around and, hey, might've been created with a Dawnshard's influence anyways). Third thought is that Honor, at least, knew or implied earlier in the series that he was aware of a Dawnshard on Roshar (and based off no Shard noticing Rysn/Hoid having one, clearly they can't simply sense there's one on their planet). So how did he know? Because someone used it once. Or tried to. In my eyes, that all points to it being the Change Dawnshard which 1) we know is/was on Roshar, 2) probably wouldn't effect Cultivation (way too similar to her intent), and 3) it fits some of the missing pieces.

This just strikes me as Hoid thinking/acting like he's more clever than he is. He used to have the Dawnshard, gave it up at some point, and then reacquired it, so he seemed to think "oh, no one would suspect me of having it now!". Which, fair, worked, but still. I don't think it's anything more than that. Although, maybe he had some backup plan involving it and what he could do with it once he fully bonded Design.