Random event appearance

Hey im currently making a free roam zombie game. i want to make those "zombie" event appearing at random place is there any script that can do this?

You need to define what "random" means.

Should a zombie appear in water?

Also, should you be able to permanently clear a map, or should the events perpetually re-spawn? And do you want the zombies to actually be events on the map [roaming around] or will it be "sudden-encounter"?

Also... this can be accomplished with events.