It could be anything from the monster approaching or coming into this reality or however it got here to a field it creates to just casting some kind of spell. I wish I knew the specifics, but it sounds kind of like the shadows my team and I fought at home. Shortly before a big one showed up at the full moon, there'd be an increase in a disease called Apathy Syndrome among the populace.

[Only killing those big shadows didn't work so well, did it?]

We'll have to see if there's a next time and if it pans out the same before I can make any concrete theories about it compared to home, though.