This idea came from seeing names like Balance and Sacrifice suggested both for the Nightwatcher, and for the Silence Divine Shard. (I like the name Sacrifice more than Balance.) We know almost nothing about either of them, but they do feel similar. Also, the Silence Divine planet is in the same solar system as Roshar.
So I think this Shard fled Roshar to the other planet, and the Nightwatcher is a splinter it left behind.
Also, the price paid by the Oathpact is pretty terrible. It may involve the power of Sacrifice, not just Honor, or maybe not Honor at all. Honor's principle seems to be that you not do horrible things in the name of good, and asking the Heralds to submit to torture is pretty nasty.
One thing I can't work in, is that if there's any fleeing of Shards from one planet to another, shouldn't they be fleeing to Roshar? That's what the myth of the Tranquiline Halls suggests. I don't know what to do with that.
edit: I just learned Eri had basically the same theory a few months before I wrote this. It's nice to see I'm not the only one who thinks up crazy stuff like this!