User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-50.10.112.112-20121130212724

I think the oath 2 keep w/ final breath is the one Nico made 2 Percy b4 Percabeth fell 2 Tartarus. The sacrifice Percy can't make (according 2 me & my brain until RR comes out w/ HoH) is Nico going through the Doors of Death 2 close them on the Tartarus side so Percabeth can escape.