At the end of The Battle of the Labyrinth, we see Nico suddenly appeared out of thin air.
         " Nico taped his silver ring. "But that's not the real reason I've come. I've found out some things. I've come to make you an offer."
         "What?"   
         "The way to beat Luke. If I'm right, It's the only way you'll stand a chance."
         I took a deap breath. "OK, I'm listening." [BOL page 361]  "
Then Nico get's distracted by the blue birthday cake, and the book ends.
    You probably were so excited and sad that the book was over that you missed this important bit of info. Nico has a plan, and it might work.
    We are reminded of this by the author in The Sword of Hades [The Demigod Files, Page 133]. If this isn't important, Rick wouldn't mention it more than once.
    The plan will involve the underworld, since Nico is a son of Hades, and Hades is the god of the Underworld. We can be shure that Nico wil be conjuring alot of dead in book five.
    Rick Riordan had confirmed that we will be seeing Luke's mom in book 5, but why? Well, we do know that Luke ran away from home, thoe we don't know why. We can only quess that what happened was simular to what happened to Annebeth (She ran away from her parent because he married a second wife that she didn't like.) of Thalia (she ran away because she didn't like her mom, who was mean to her, even thoe Thalia loved her mom.). Lukes mother could of been in thoes two situations.
    What also could have happened was that Luke's mother was killed, and Luke ran for it, eather to escape the authority's or to escape the Orphanage.
    Luke's mother is dead. We don't know how she was killed, but w\e know that she is dead. If she wasn't dead, then Nico wouldn't be involved at all, since the dead are his speciality.
    Luke loved his mother (you love you're mother no matter what/who she is), and It is possible that Cronos bribed Luke into his service in the promise of getting his mother back. Or, Cronos could have made an ilusion of his mother saying something along the lighns of (join Cronos for me.).
    Nico's plan is to do the same thing Cronos did, bribe Luke with his mom. Or Nico could just conger Luke's mother up, and Lukes mother would tell her that she loved him, and to abandon Cronos. Luke would then do this because he loved her (you love you're mother no matter what/who she is).'  
    Even thoe Cronos inhabits Luke's body, Cronos is not under complete control. Rachel managed to bring Luke back temporarily by hitting him with a plastic hairbrush. When Luke see's his mom, that will be enough to get him out of Cronos' control, permenetly, or long enough to tell percy something that will defeat Cronos (a piece of info, where a weapon is hidden, ect.).
    Hestia comes in because she would help Nico because his plan does not involve violence. She would assist him and Percy in every way she can. Even thoe It does look like Percy wont be using Riptide allot, He will still have to use it to distract Cronos.