Rahyeh dashed out of its arena after doing its edge to edge dash. I found it back at the entrance of the instance, unable to move or attack (but able to be killed).
The attached zip is just a screenshot of the situation.
Apologies, but unfortunately, no. I think I was standing in the top right quarter of the ring, and that he dashed to the bottom left quarter, but I am not 100% certain.
…playing with Volatile Reversal makes it pretty hard to know where you are