Hades' Sword (also known as the Sword of Hades) is Hades' main weapon. It is made of Stygian Iron and contains one of the Keys of Hades. The sword allows the wielder to send anyone to or from the Underworld with the lightest touch. Because Hades already has a symbol of power, the Helm of Darkness, the power of the sword could potentially tip the balance of power in Hades' favor over Zeus' Master Bolt or Poseidon's Trident.
Persephone, the Queen of the Underworld as well as the Goddess of Springtime, recruits Percy Jackson, Thalia Grace, and Nico di Angelo to help recover Hades' newly forged sword which has been stolen. She locks the realm while they head out in search of the lost weapon. The trio of demigods face Melinoe, the Goddess of Ghosts, and encounter Ethan Nakamura and the Titan Iapetus. They retrieve the sword from Ethan, who escapes, and return it to Persephone. But Thalia protests about Hades' new weapon of power but Persephone silences her and thanks Percy and Thalia for making their uncle more powerful. However, Hades is angry as Persephone went behind his back and ordered the sword to be made to make her husband the most powerful of the Big Three. Hades swears an oath that he will not use this sword against the Olympians.
Hades is seen with a blade of Stygian Iron as he combats Kronos' army. It is unknown if this is the same sword mentioned in The Demigod Files, but since Hades did not usually fight with a sword in the past, Nico mentioned his father uses a staff in battle, and he fought with a staff against Percy, it is probably the same weapon. It appears that he has abandoned his staff for his now-finished blade.
The Sword of Hades can send and release anyone to the Underworld with the slightest touch. Due to having a Key of Hades embedded in it, just touching against someone can send them to the Underworld, making it easy to defeat foes, as you don't even have to penetrate the skin to kill them. It is likely that Hades can channel his power through it, using the Key.