many times what happens and this happened to me in the quest where I had to clear the library of monsters. Sometimes you may hit a monster or in this case a balverine *in my case it was a hollowman* out of the screen. Make sure that you don't see any balverine that is out of screen, meaning walk the perimeter and double check. If you see him then of course a simple spell or attack ought to do the trick, if that's not the case then unfortunately there is a glitch which the game company has made noticed to in yahoo news that there have been a few...sorry but try my suggestion first.

kill the guy who turns into the white balverine

Might be a bug... You 100% certain theyre all dead?