Set during the winter between the fourth and fifth books of the main series, this short adventure throws together the three living children of the eldest gods: Percy, son of Poseidon; Thalia, daughter of Zeus; and Nico, son of Hades. Summoned to the Underworld under false pretenses by the goddess Persephone, the trio learns that a powerful new sword has been forged for Hades and then stolen by a demigod spy. The weapon is no ordinary blade; bound into it is one of Hades's keys, an object that can open the boundary between the living world and the dead. If it falls into the wrong hands, the dead could pour back into the upper world. Reluctant allies with very different loyalties, the three demigods descend through the perils of the Underworld to recover the sword before its theft can be exploited. The story is a compact, fast-moving side quest that brings the offspring of the Big Three together for the first time, deepening their uneasy dynamic ahead of the war's final chapter. Originally published as part of a companion collection, it offers extra adventure and character texture for readers of the main saga, trading the larger arc's sweep for a tightly contained mission underground.