This event is harmless if you are not near any of your tamed animals but is pretty brutal otherwise. It spawns about 8 bats and the ones that are killed respawn near instantly. They have no player priority targeting. They go for the closest entity with a health pool. All of that combined with the fact that each bat does a hit and run tactic for 20 dmg means that your livestock is doomed if it isn't sealed in a pen. I have lost a group of seven 1-star wolves to this invasion.