Nico Di Angelo's fatal flaw is holding grudges, as Bianca tells him. In his case, it can lead to not wanting help or not asking to be saved in deadly situations because he does not like someone. Examples are his not wanting to be helped by Percy, even when Geryon was going to sell him to the Titans and not wanting to go back in the Labyrinth with them. Bianca shares this with him, as she also had the same fatal flaw in life, although this did not lead to her death. Bianca also says that holding grudges is the demigods of Hades' fatal flaw. In the fifth book, Nico tells his father, Hades, that he is holding a grudge that could end up killing the Olympians and the campers.