I know he's meant to be a mysterious character in the lore and I was reading through my recently acquired Book of Cain (hooray!), when I came upon the nephalem Rathma. From what I've read on these forums about Trag'Oul and the description of Rathma in the Book of Cain, I can't help but wonder what connection these two characters have with each other. There is a note in the book stating the necromancers who followed Rathma depicted him in the form a great serpent, which is also the only form I've ever seen Trag'Oul depicted as. Both characters seem to share the same concerns from a universal perspective and (correct me if I'm wrong) Trag'Oul assists/ed or teaches/tought necromancers.
Did I miss some lore where it is explicitly stated that these two characters are one in the same, or is it only implied in the Book of Cain? Or am I completely wrong in this assumption?