After the "truce" was brokered, demons have been a fairly rare sight on the surface. However because of how vindictive they can be, and how long they live, occasionally one will show up after decades to "enact vengeance" against people or places for wrongdoings, perceived or otherwise. This means that it is EXTREMELY important for humans to maintain defenses and alliances against this threat. Demons are not only very powerful, they are often cunning and capable of waiting for a moment of weakness in order to strike. A Walled City that can easily defend against roaming monsters can be hit hard by a seemingly much weaker demon.
It is extremely rare for numerous demons to appear in one area at the same time, and if they do it is much better for the humans in the vicinity to flee rather than fight. A response will be triggered to help push them back to where they belong, but it can take days for the forces to be approved and sent out.
Again, unless you can fight for days, it is better to FLEE.
-a many years old missive to the leaders of the Walled cities from the Angels