User blog comment:Hawki/Legacy of the Void - Reflections, Ruminations, and an End of an Era/@comment-4817-20151207030139/@comment-58609-20151207033917

It's in the very last mission. I forget the exact line, but Amon addresses Kerrigan, stating (paraphrased) "this transformation was forced on me, as it was on you" (in reference to Ouros's essence transfer).