Spontaneous Monsterficiation -prob change name-

Through the powers of the evil goddess Wenna those truly lost, desperate, broken and twisted can spontaneously turn into monsters.  The exact way this manifests is unknown but from what the very few survivors can remember at there lowest point they here a voice whispering to them offering them a deal. A deal that will end there suffering and grant them immense power to finally take control of there life. 
In practise this phenomenon means that theoretically anytime, anywhere somebody can turn into a monster. This has lead to widespread paranoia especially against social outcasts. Social outcasts are the most likely to turn into monsters so there often run out of town completely.
When a monster does appear, if there is no one around to stop them they tend to rampage and kill the entire village they spawned in, afterwards they can't find enough food to sustain them since they need a very large amount of it and starve before they can reach any other settlement