Heroes of Olympus 4: House of Hades

Gaia, Mother Earth, has decided to destroy the mortal world with her
troops of monsters and giants, and there is only one way to stop her:
to close the Doors of Death, through which these creatures are escaping
from the underworld. And, most importantly, it must be done from
the outside... and from the inside.
Hazel, Nico, Piper, Leo, Frank, and Jason have discovered that the outside
is in Greece, at the temple of Hades, god of the underworld, although
they have a long journey to get there and do not know what they will
have to face when they arrive.
Meanwhile, Percy and Annabeth are somewhere on the other side,
fighting for their survival, to find the doors... and to get out before
it's too late.