User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-66.30.249.83-20121002153937/@comment-98.161.31.153-20121003115603

SPOILER:

At the end of the book, in arachne's lair, Percy and Annabeth fall through to Tartarus. Percy tells Nico to lead the others to the mortal side of the doors of death(the other side is in Tartarus) Then the book ends