The problem in *The Last Olympian* is the threat of the Titan Lord Kronos, who is trying to overthrow the Olympians and rule the world.
Here's a breakdown of the main conflicts:
* Kronos's army: Kronos has amassed a massive army of monsters and Titans, who are attacking both Camp Half-Blood and New York City.
* The Olympians are weakened: Zeus has been imprisoned, and the other gods are struggling to defend themselves against Kronos's forces.
* The prophecy: A prophecy foretells that a demigod child of the Big Three (Zeus, Poseidon, or Hades) will either save or destroy the world.
* Percy's role: Percy Jackson, son of Poseidon, is the chosen demigod prophesied to play a key role in the final battle against Kronos.
Ultimately, the problem is the clash between good and evil, with the fate of the world hanging in the balance. Percy and his friends must fight to stop Kronos and his army, save the Olympians, and protect humanity.