Thalassa, the Ocean Devourer.
A divine being that served as the mount of the Ocean Titan, Khion.
During the Age of the Titans, it was a being second only to the Titans themselves. It could traverse the oceans in minutes, and anytime it moved its massive body, tsunamis and hurricanes were sure to follow. Compared to the average human, it was pretty much a god.
Possessing enough mana to drown entire cities, it was a living calamity that brought destruction just from moving.
That's why its awakening was so dangerous.
If it were allowed to roam the world just as it did during the Age of the Titans, half of the Hyades Republic would be torn apart. Without the Titan overlords to reign it in, Thalassa would do whatever it wanted. And the result wouldn't be pretty.
It would sink every continent with its massive oceanic abilities, causing the end of human civilisation as they knew it.
That was why it was given the title of the Second Calamity.