This is a rubber Stunt Sword used in the epic Zack Snyder movie '300'.

The sword features battle weathering and a leathered handle; it came with its own scabbard marked "Stelios DBL" on the belt, meaning it was either a backup piece (double) or used by Michael Fassbender's stunt double on fight scenes, hence, screen used.

COA: Hollywood History

Image 2: Michael Fassbender as Stelios in a moment of the film where this sword can be clearly seen.