Nico di Angelo
Nico's sword is made of Stygian Iron, which will kill monsters, and, unlike the other metals of Celestial Bronze and Imperial Gold, is capable of harming mortals. The metal, when being forged, is cooled in the River Styx, making it indestructible. It is implied that only beings of the Underworld, such as Hades and his children, are able to use a Stygian iron weapon. The sword helps him channel his power to control and summon the dead, along with draining an enemy's soul to reinforce his sword's power. He can also use it to create fissures in the ground, trapping anyone unlucky in the underworld. This is like what he did to king Minos. Nico being an incredibly powerful demigod; he has many powers and is aware that “With great power... comes a great need to take a nap," as he says after putting to sleep all of the skeleton guards in the Palace of Hades during his and Percy's escape from imprisonment during the Last Olympian.