Deathstroke's Sword was a weapon used by Slade Wilson aka Deathstroke. Deathstroke seemed to use it as his last resort, as evidenced by both of his boss fights in Arkham Origins and Arkham Knight. It was not available to the playable version of Deathstroke.
Arkham Origins Incident
After Batman destroyed his Bo Staff, Deathstroke opted to use his sword in an attempt to take him down. Deathstroke was much faster with his sword than with his Bo Staff. Batman stopped Deathstroke's final attack with his gauntlets, disarmed him, threw the sword into a wall, and bashed Slade's head into the hilt, which rendered him unconscious.
Arkham Knight Incident
After Batman destroyed his tank, Deathstroke leaped out of it, with his sword in hand, ready to fight. However, while Deathstroke was still in the air, Batman took him down so fast that the sword was seen falling to the ground even after Deathstroke had disappeared from the screen.