Um, I skimmed through the whole thread, there are many interesting train of thoughts going on (especially those detailed and complex ones about time travel), but in the end, I still think what I had thought before: the assasin is just an assasin, someone we haven´t met until now. Why he wanted to kill Foltest, who sent him, and why he is mutated are simply a teaser for the following story. We can speculate about these question, but from what we know at this point, we can´t reach any definite conclusion as the mystery won´t be solved until the continuation of the story. The assasin is not Alvin as Alvin´s story has already been resolved: he was the Grandmaster, who pulled the strings from behind the scenes and in the end emerged as the major bad guy of the game. We learnt what his agenda was, who he was, what was his relation to us - and we killed him, and thus closed the story arc of the game. His story is over, he has no further purpose.The assasin is not Geralt or Berengar, either. Simply because all the witchers we know come from the school of Wolf and they all have the same fighting styles and use only one sword. There is no way to refute the theory that the assasin is Geralt´s evil twin or something like that who underwent the training and mutations in Zerrikania and became the first zerrikanian mutated assasin, but that´t imho too much of a stretch.