What is the conflict in the red pyramid by Rick Riordan?

Carter(14) and Sadie Kane(12)'s father was summoning Osiris so he can bring his wife back from the dead. Instead of just summoning Osiris, he summons the gods of the Demon Days, including Osiris, Iris, Horus, Nephthys,and Set. Since gods need a human form as a host, the other humans that were there were Julius(their father), and the kids. Osiris took on Julius's form, and Set took him in a coffin that sunk into the ground. Since the old legend is that Set tricked Osiris into going into a coffin and never coming out, this is what also happened in the modern day. Set started building a red pyramid and put Osiris's coffin into it so he can never get out. Now the Kane siblings, now hosting Isis and Horus, must learn how to use their powers, to defeat Set, and save their father.