Iron Forged in Fire: Part One - S1-E1
Continuity mistake: On his first flight with the Iron Man armor, Tony loses control trying to avoid a building and falls towards the road. As he is regaining his flight, the car he is heading right at is a gray vehicle. It then cuts to a close up from the reverse angle as Tony avoids the car and it suddenly becomes a yellow taxi as it swerves and stops in the road at an angle. Then the shot changes again as Tony is flying away and now the taxi has turned into a purple car. (00:16:40)
Chosen answer: It would depend on four things: the sharpness of the sword, the strength of the steel used to forge it, the strength of the man swinging it, and the luck to slice though one of the ligaments between the victim's vertebrae, but theoretically, yes, it's possible to sever a head with a single sword blow.
Captain Defenestrator