User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-65.188.251.66-20121009025606/@comment-186.82.70.150-20121010002526

i read a mithology encyclopedia, and i discovered that the owl, athenas sacred animal, has the power of going in and out of the doors of death at free will, so this is what happens, jason and the others finally make peace between greeks and romans they charge at the doors where they close them from their side, on tatarus percy and annabeth close the doors too, that means that they stay behind, being pretty sure that they will stay there forever and die.

but someone really smart (probably leo) manages to figure out how the athena parthenos works, dissolving all tha natural hate between the two demigodly sides, and returning athena to her former self, realizing the danger in which her daughter is she goes across the doors and rescue annabeth and percy (obviously not liking to save percy too but doing it anyway), and like that everyone is safe.

as for the oath with the final breath, nico promised percy to guide the rest of the seven to the doors so he probably dies doing that