Copy and pasted from a reddit post by a redditer named "Builder"

"FOR THE LOVE OF GOD PEOPLE, COVER YOUR BOAR PENS. I'd only just started exploring the frost caves when I went to my boar farm and a "You Stirred The Cauldron" event triggered. I had over 20 two-star boars and piggies in a simple pen. The bat swarm descended. Not only are they virtually impossible to hit, there are so many that there's virtually nothing you can do before they kill your entire herd. I'm now left with one nearly dead piggy. I'm turning that boar pen into a fortified ♥♥♥♥♥♥♥ castle now."