Who was that person who visited Kosoter in the TJE prologue...?
I have seen convincing argument from HP here somewhere that it was Sarl. By use of phrases he uses throughout.
Really? I would like to see that argument. I looked back at that scene, and the text is quite clear that the traveler is new to the Skin Eaters. He speaks as 'we', and knows about the nickname, - Ironsoul -, Kellhus gave Kosoter during the Unification Wars. He says he's looking for the one who is called 'Ironsoul'. Sarl was there, or at least he says so to Akka, when Kellhus named Kosoter Ironsoul. That does not seem to be the same person.
The 'we' implies that the traveller is an agent from one institution or another. I guess the Empire. And I furthermore think now, as others have suggested as well, that it is indeed Akka's slave, Geraus. As we later learn that he has prepared Akka's meeting the Skin Eaters. Geraus could very well be an agent from the Empire, keeping a close eye to Akka.