In most fantasy stories, and even IRL, there are horde-like nations/beings/creatures that come from a single direction (i.e. the North).
I am creating a world where there is a demonic portal spawning straight in the middle of the known world. Ideally, this center of the horde would be difficult to simply bullrush into by other nations, though not impossible.
The main question is:
How would medieval societies likely react to the presence of a large horde coming from the exact center of the known world?
Would they try to reach people on the other side of this gigantic horde circle? Would the world be separated? Or would they adapt in some other way I likely am not aware of? Assume also that before this hordelike group appeared, the known world had been in contact with each other.