The entity Gavilar spoke with in the prologue did not sound or behave like the Stormfather despite being called that. I have finished WAT and still remain unconvinced that this was indeed the Stormfather. We know Odium was also searching for a champion, and thats one of my theories of what it was. I also thought it could be Ishar seeking his own champion, which would tie in very nicely with the end of ROW and his madness. I know the Stormfather/Tanavast admitted to speaking with Gavilar and pointed out his faults but there was no direct reference to the prologue.
Also, there is an exchange in the prologue which states - "You must take this, Gavilar said to the Stormfather. They must not get it Tell..my brother..HE must find the most important words a man can say..." "No, the Stormfather said, tough a hand took the sphere. Not him. I'm sorry Gavilar. I made that mistake once. I will never trust your family again" - well we know the Stormfather picked Dalinar anyway, and we know he knew Dalinar is a good choice.
So What is this thing speaking to Gavilar?