Mod Name: The Mob Invasion Mod (or something like that)
On some nights, there is about a 5% chance that there will be a specific mod invasion. This is where all other hostile mobs do not appear, and only one type of mob spawns. For example, if there was a Skeleton Invasion, only skeletons would appear for the entire night. If there was a Zombie Invasion, only zombies would appear. The type of mob invasion that occurs could also be determined by random chance.