Hades' Sword

The Sword of Hades is Hades' new weapon. The sword is made of Stygian iron, with the one of the Keys of Hades in its hilt. The sword allows the wielder to send anyone to or from the Underworld with the touch of the sword. It is said to be a potential rival to Zeus' Master Bolt or Poseidon's Trident. Like Hades, all known demigod children of Hades presently use Stygian iron weapons.

The Demigod Files
In The Sword of Hades, the goddess of springtime and Queen of the Underworld, Persephone, recruits Percy, Thalia, and Nico to help recover the sword because it had been stolen. She locks the realm while they head out in search for the lost weapon. Their journey was perilous as they battle horrible ghouls and encountered an old enemy, Ethan and the Titan Iapetus (who Percy dumps in the River Lethe and says Iapetus is named Bob and fights for the gods). With their combined strength, they are able to combat the evil and gain a new ally in the process, but at a cost of Hades receiving a new weapon. Thalia protests about this later, which Persephone stops, due to the fact that she had ordered this sword to be made. Hades had not wanted this sword, but either way, it was made.

The Last Olympian
Hades is seen with a blade of Stygian Iron as he combats Kronos's 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 one against Percy) it is probably the same weapon.