The deepest region of the Underworld, a primordial abyss that is both a place and a being. Monsters that are killed reform in Tartarus before eventually returning to the mortal world. In The House of Hades, Percy and Annabeth fall into Tartarus and must survive its nightmarish landscape - rivers of fire, poisonous air, and monsters in their most primal forms - to close the Doors of Death from the inside.