In The Lost Hero, Gaea freed Enceladus from Mount Diablo, and Enceladus uses the mountain to imprison his enemies like Tristan McLean. Jason, Leo, Piper, and Coach Hedge went to Mount Diablo to free Piper's dad. Jason killed Enceladus (with help from his dad) and re-trapped him in the mountain.
The Native Americans of the area lost a battle against the Spaniards there earning the mountain its name, the Devil Mountain (in Spanish). It is considered a cursed yet holy place for them. The Natives that used to live there are the Chumash.