Black Monster
The Black Monster is a mythological figure found in historical texts and religious stories. In the Prairie, Dart attributes it to the destruction of his hometown of Neet and the reason for his five year long journey for revenge before returning to Seles. In Serdio most information related to it was reduced to ashes as even the Minister could only recount that it was the devil that disobeyed the great existence, and was known for devouring the god. In Tiberoa, more of its history remains intact as Fester, an astrologist and Governor, was able to recall that its appearance coincided with the descending of the Moon Child from the Moon That Never Sets every 108 years. In Mille Seseau, the mythological context is revealed: the Moon Child is sent as a herald of the gods to revive them on earth only to be slain by the Black Monster who abhors it.
Story Revelations (Spoilers)
In Vellweb is is revealed that the Black Monster is real and has been massacring villages for thousands of years to kill every Moon Child that has been born. When Rose is revealed to be the Black Monster, it is also exposed the Moon That Never Sets is actually the unborn flesh of the God of Destruction: the 108th fruit of the Divine Tree. The Soul of this God transmigrates into a newborn Human every 108 years and that is what it means to be the Moon Child. It is said to possess a power that turns those around it into evangelists for the God, which Rose states is why the massacres have been necessary. Killing the Moon Children has prevented any chance of the Soul from reaching its flesh and being born.
In Ulara, Charle reveals that Rose has saved the world 107 times. Including the 18 years since the last Moon Child was born, this means the first Moon Child died 11,574 years ago. Since the Soul of the God could only begin transmigrating after the Crystal Sphere was shattered at the end of the Dragon Campaign, the Dragon Campaign must have ended first. However, whether it took 108 years for the Soul to begin transmigrating or began immediately after being freed is unclear, so while an exact year cannot be deduced, the Dragon Campaign must have ended between 11,682 and 11,574 years ago.