The Heraneion hidden underground.
The Heraneion is a grimoire in Campoine.
The Heraneion is a large grimoire discovered by the Bronze-Black Cross sometime before the start of the series. It held the power of Hera, an Earth Mother Goddess. When the Heraneion was destroyed, the power that was unleashed became a Divine Beast in the form of a dragon.
After its discovery, the Heraneion was hidden in an underground chamber for some time while being kept under control by local witches. After the discovery of the Gorgoneion, it began to react and steadily became more active over time. Fearing a catastrophe, the Bronze-Black Cross asked the Campione Salvatore Doni for help, only to have him cause the catastrophe they were hopping to prevent by cutting the Heraneion to pieces and releasing its energy which took the form of a the divine beast (dragon). This power was eventually absorbed by Athena, after it was attacked by Perseus.