User blog comment:Menara/The Son of Sobek: The official PJ and Kane Chronicles Crossover/@comment-75.172.142.247-20130517193838/@comment-3146930-20130517221830

It kind of is. I mean pretty much every major event in the series so far has been driven by the Doors of Death.
 * The Lost Hero: As a reward for helping Gaea, Medea tells her about Leo's role in the Prophecy of Seven. Gaea kills Leo's mom and Leo hates Gaea. Anyway, when Gaea takes over the Doors of Death, Medea returns and uses her charmspeak to get Piper's dad (using Jane) captured by a Gigantes.
 * The Son of Neptune: The quest is literally to find Thanatos so he can stop souls and monsters from escaping using the Doors of Death; as without him people are just coming back to life and monsters aren't staying dead. Not only is the goal to free Thanatos, but find out where the Doors of Death are.
 * The Mark of Athena: After learning Nico had been captured, one of the major goals is to find Nico before he dies so he can tell them the location of the Doors of Death...which he does. And that brings us to...
 * The House of Hades: Percy and Annabeth need to make it to the Doors of Death to get out of Tartarus while the remaining group have to get to the mortal side of the Doors of Death so they can close them.