The topic that is brought up during Season Unending.

Do you think it's too ambiguous to make a guess, or do you have any opinion on who's guilty or innocent?

That actually really thew me, because outside of that peace council, I haven't heard a single thing about that apparent massacre.

All I could find about it is that either side could be accused of the massacre. Could even be that the massacre was originally suppose to be the Markarth incident because originally Karthwasten was suppose to be a large city and the capital of the Reach. But who knows, at most I'm going with Bethesda forgot to remove something that was originally meant to be cut.

I thought it was just a throw-away line so that one side could accuse the other of an atrocity and demand justice from it.