Folks may know this from JaWoodle’s “Anywhere, Anytime” series. It creates random wandering hordes, of a random amount of zombies, a random amount of times per day. How often (in terms of in-game hours) and how many zombies are configurable in the blocks.xml included in the mod.
This is a C# Mod (cannot be server only, EAC must be off!).
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The forum topic of the mod is here.