Unfortunately, the only insight I have is that I don't have any insight. Magic did not appear to have been used in the actual murder of the Saxons, though I cannot be so certain about the cleanup and transport of the bodies afterwards.

It's still possible that the perpetrator had some kind of invisibility spell or something so that they could sneak up on them. Specifically, I find it difficult to believe that someone just walked up and stabbed the Master from the front without him noticing them, but if such a spell was used I would not have been able to detect it after the fact.