Commentary Purg IX 82

The warder's sword is 'the sign of one who has authority to pronounce sentence,' in the words of Singleton (DDP Singleton.Purg.IX.82), citing Benvenuto da Imola.  Singleton goes on to add that it is also 'reminiscent of the Cherubim with the flaming sword that were placed to guard Eden after Adam and Eve were driven forth.'