Build in a psychic drone message basically giving the player a heads up (at least 1 day in advance).
The next day spawn in X amount of low health melee rushers.
If possible have the initial rushes be a low amount of units and progressively add more each time.
It's not really my intention that they should be able to over come a reasonable defense it's just a fun diversion.
They should be butcher-able and/or drop some crafting materials.
I really wanted the zombie mod but I see it's not updated for Alpha 8, maybe someday! :)
Creating a zergling swarm attack wouldn't be difficult at all. The one-day in advance is questionable though... can't say I know if that's possible with the current system. I would jump on this but I'm currently way behind on my own mod right now :(
Edit: After looking at battle.net, I found that they have 3-d models of each unit, which would make this mod even easier to make than I originally thought. Perhaps, if no one else picks this up any time soon, I can take a dive into the idea of Zerg invasions :)