Era beginning/end
The day that marked the potential for civilizations to sustain themselves.
The Ancient Seas has always been a power struggle. Battles between Sea Monsters would destroy anything in their wake, making it impossible for civilizations to survive more than a few decades, or if they were lucky, centuries. This chaos reached its most terrible point during the existence of the Big Three: the most powerful Monsters the Ancient Seas have known. The most powerful was The Elder Hag: a creature of raw magic and malice who created many other Sea Monsters, hoping that they would grow strong enough to give her a good fight, but none ever satisfied her. The second, was Lord Fear (as named by the Selkies), a colossal orca whose was not only physically dominant, but his presence could drift into the mind of the sleeping and murder them in their dreams. The third, was the witch Agathys. She was much less powerful on her own than any of the others, but was able to grant magic to civilizations who pledged allegiance to her, and with those numbers, her power was fostered and grown. After centuries of Agathys' civilizations growth, she decided to challenge Lord Fear so she could rise to the #2 position. What she did not know is that her civilizations had found out about her treachery: she intended to sacrifice them all and absorb the power that had grown within them, so she herself would be strong enough to take out Lord Fear and perhaps even rival the Elder Hag. The civilizations worked together in secret and when the day came and they were face to face with Lord Fear, they sucked the power remaining in Agathys out, and used it for themselves. This crippled her permanently, and she faded away. The power they channeled from her along with their own was enough to critically injure Lord Fear and force him to retreat, a fraction of the power he once had. The Elder Hag watched this conflict and mournfully understood that the extent of the power of the creatures who were supposed to be just under her was so pitiful. So she tore open a rift into the Elemental Plane of Water and left this reality behind. From that day, civilizations were able to thrive. There was still the usual chaos of the Ancient Seas, but since then, there have been no Sea Monsters that have been able to monopolize power in the same way the Big Three have. Skilled Civilizations were able to easily exist for centuries and sometimes even for millennia.