While it is not mentioned in The Last Olympian the Battle of Mount Othrys was the final engagement on the Roman demigod's participation of the Second Titan War, which occurred at the same time as the Battle of Manhattan. The Roman demigods, commanded by Jason Grace, son of Jupiter, assailed the Titan fortress of Mount Othrys, confronting an army of a few hundred Scythian dracanae and the Titan Krios.
While there is no mention of Camp Jupiter's legion, Jason Grace or the Battle of Mount Othrys until The Lost Hero, it was the demigods of Camp Jupiter that battled what little of the Titan Army that had been left to guard the base. The only allusion to the fall of Mount Othrys came when Percy asked Hermes what happened to the Titans and their palace. Hermes lied telling Percy that when Kronos was destroyed the Palace self destructed and when it fell apart Krios had gone in to hiding (when in fact Krios had been destroyed by Jason Grace with his bare hands and the Roman demigods had destroyed the palace and the Scythian Dracanae).
While discussing how Mount Othrys collapsed as a result of Kronos' defeat at the end of the Battle of Manhattan, Jason begins to remember the battle, and at one point proclaims "I toppled the black throne of Kronos and destroyed the Titan Krios with my own hands!" While Titans are immortal and thus cannot be completely destroyed, it is presumed that Krios' physical body was destroyed and his essence returned to Tartarus. Jason also remembers flashes of the battle: a giant in starry armor and a helm mounted with a ram's horns. He remembers an army of Roman demigods scaling Mount Tam, fighting through hordes of snake monsters. When he reaches the top, he defeats Krios and destroys the throne, unknowingly aiding the Greek demigods that are protecting Olympus in Manhattan.
The Son of Neptune
The Battle of Mount Othrys is mentioned again in The Son of Neptune. Nico tells that the Romans attacked Mount Othyrs and Kronos (whom he mentions as Saturn) faded into the abyss when the palace was destroyed.