Potential Oathbringer Spoilers
So this is very much a guess, and I doubt it's true. But could there be another Shardholder on Roshar, influencing Dalinar?
We know he's had visions that do not have to do with Honor's influence. We know that whatever Dalinar became, Odium was certainly surprised by it. And at the end of Oathbringer, he saw a light that Navani couldn't.
If this is true, could there be a sort of Enigma or Mystery Shard attempting to work against Odium? One whose influence is far more sublte than Honor or Odium? Maybe a Shard that held such secrets and power that Odium teamed up with another Shard (hence "we killed you"), to attempt to Splinter. Maybe he was talking to this specific Shard when Dalinar declared "I am Unity".
Once again I doubt this is true. My current belief is that some form of Adonalsium is influencing Dalinar, especially with the "Unite Them" repetition for him. But what do you guys think?