War of Remnant
The war amongst the Kingdoms of Vale, Anima, Atlas, and Mistral caused massive catastrophe across the globe. But the worst of it wasn't the war itself but what the war gave birth to. The soldiers who fought especially the best of them became battle hungry and never loss the appetite after the war was over and supposed peace was brought about. These soldiers either became bandits, mercs, or enforcers in mafia. And what's scary is that these soldiers learned to kill in very scary ways, had a profound understanding of aura, and their experience is nothing short of ludicrous. Coming across just one is like running into Death itself.
Though these soldiers weren't the sole product of the war. The abolishment of slavery and encouragement of creative freedom looks good on the outside but this created shady competition and markets doing the very thing that was now criminalized. The Underworld was born. And the rise of crime was simply a byproduct to a seemingly good thing. Life was made different for everyone indeed but not everyone liked the idea of change. Of course the peace brought to Remnant also brought about the creation of a new force of hope. Huntsman and Huntresses. Men and women sworn to serve Remnant in the hope of giving hope to the innocent in a seemingly never ending war against the Creatures of Grimm.