If I were just using my observations about the Cosmere to think of an answer I'd say: Autonomy Avatar seems really likely, or maybe it's just a bunch of loose investiture from Ambition, Mercy, Odium from their fight, or maybe it's Taravangodium hiding that it's him.
If I were answering based on my knowledge of epic fantasy then I'd say it's the first glimpse of the forces of Anti-Adonalsium. Anti-light has to be Anti-Investiture, which has to come from somewhere. It only makes sense that Adonalsium, whatever it was, had an enemy, or an opposite. The devil to it's god. We haven't seen any direct evidence of this yet, but if you look at the Cosmere as one titanic epic fantasy series, then it makes sense that the biggest baddest bad will be something larger in scope and more powerful than a mere Shard.