I haven't seen any theories about The Rift/Rathalas yet, so I'll just throw mine out there:
Based on the map of Roshar, Rathalas is in ambiguous territory. It could be part of The Eastern Crownlands, or Thanadal's Princedom, OR the Kholin princedom. My theory is based on viewing it as part of Princedom Kholin. If it is the latter, it wouldn't make sense, in terms of Gavilar's war of unification, to have a battle there--it's already part of his princedom. But consider this WOB (copied and pasted from another thread):
Brandon: "Gavilar was 4th dahn before becoming highprince, for example. His branch of the Kholin family wasn't considered a prime contender for the highprince throne--until he took it for himself."
Question: "His branch of the Kholin family? Does this imply there are other branches of the Kholin family? Meaning, there are other Kholins elsewhere?"
Brandon: "Well, not as many as there once were..."
I was trying to come up with something that was so terrible that it effected Kadash, a seemingly hardened and tough warrior, so strongly. Maybe Rathalas was where Gavilar and Co. wiped out the other Kholin branches.