Population Migration / Travel
Those of the Sicatrin Empire looked down upon the monsters, thinking them stupid, evil creatures only fit to be slaves housed in grimy prisons. So when the monsters did not quickly move to Tsaraknit, the chosen prison for them, the Emperor began a concerted effort to trap each one of them behind the mountains. When 20 years of this effort had passed, every known monster was behind the mountains, doomed to spend the rest of their days as slaves.