It is the site where Mount Othrys was being rebuilt by the newly freed Titans as a war base during the Battle of Manhattan. Kronos, the Titan Lord of Time was assaulting Manhattan at the time so he could get to Mount Olympus which is on top of the Empire State Building. It is named the Mountain of Despair, and happens to also be the sight where Atlas holds up the sky. Both ordinary mist and magical Mist are very thick on the mountain. Many monsters are attracted to it. The mountain is near Camp Jupiter and close to where Annabeth Chase's father and his family live. Annabeth stayed there for the summer, saying she was able to keep watch on the monsters' activity.
Percy Jackson, Thalia Grace, and Zoë Nightshade traveled here to save the goddess Artemis and, if need be, Annabeth, from Luke's and the Titan Army's grasps. It is noted that the mountain smelled like eucalyptus, because dragons liked to chew it. Here, they fought Zoë Nightshade's father, the Titan General Atlas until they pinned him under the sky.
Percy, Annabeth, Nico di Angelo, and Rachel Elizabeth Dare, a mortal, discover a passage in the Labyrinth that leads to Mount Othrys. It is here that Percy witnesses the revival of Kronos after Ethan Nakamura pledges his loyalty to the Titan. Percy is only saved when Rachel Elizabeth Dare hits him in the face with her blue plastic hairbrush and Nico calls up the black rock of the Underworld to prevent Kronos from following.
Jason Grace mentions that during the Second Olympian War, while the Greek demigods were in New York City defending Mount Olympus and Manhattan against Kronos and his Army, the Roman demigods of Camp Jupiter were storming Mount Othrys and fighting the Titan Krios and the monsters there.