"The Hundred Monsters and the Unkind King"

Among the many myths and tales to survive from the forgotten age, the tale of "The Hundred Monsters and the Unkind King" spurs interest for its implications of an origin of monstrous species. Scholars speculate it was issued as a cautionary tale of improper behavior that utilized some real world referenced that have been lost to modern history. The origin of the myth has been lost and has been translated into many languages.

Long ago before the first age, a king sought out the advice of an oracle, fearing any challenge to their reign. The oracle channeled their powers of divination and spoke in an otherworldly voice, “When temptation is too strong, a suspicious accident shall cause the birth of monsters.”
The king feared this prophecy, selfishly, believing it meant that another would take his throne. He waged war with whoever crossed him, not in battle where he could be slain, but at his castle.
He would yell at servants who nervously made mistakes in his presence. He complained about his wife’s looks to her very face. The most egregious slight the king made was to his magical advisor, demanding he preformed magic to entertain his king, only to compare him to a court jester. The advisor was furious at being insulted so. In his fury, the advisor planned to take over the kingdom and humiliate the king; then kill him, of course.
Months of planning and co-conspiring with the many others the king attacked: the servants, the queen. The advisor was ready.
The advisor opened a portal to another world, one hundred beasts never before seen in the land had entered into the castle and eating any they saw. The advisor struggles to close the portal, fighting against a power not his own. He succeeded, at a great cost to his health. He soon realized his mistake as all around him the horrible creatures he had never seen before attacked him. Once the monster had gotten their fill of murder they all fled apart to different corners of the world.

“Treat others kindly or they will come back to haunt you.”