In the Showa series, King Caesar was the guardian of the Azumi family. He could be awakened by fulfilling a prophecy and an ancient song.
King Caesar, the ancient guardian of the Azumi family, was sleeping in a mountain, waiting to be awoken. The Black Hole Planet 3 Aliens heard of this monster, and sent their own to destroy it. As Mechagodzilla raced to destroy him, the ancient song was sung. King Caesar erupted from the mountain and took on Mechagodzilla. King Caesar fought the best he could, but he was overwhelmed by Mechagodzilla's power.