Empty mobs are a species that resemble humans with a gas mask. When an Empty Child or empty mob kills a villager, the victim will become an empty villager. They empty mobs behave like zombies, but say, at seemingly random intervals "are you my mummy" or simply "mummy".
In The Show Edit
[SPOILERS, Sweetie] The Empty Child first appeared in the renewed series' episode "The Empty Child" as a result of "nanogenes", aerial nanobots tasked with quickly healing soldiers of various species, mistaking a WWII era child with a gas mask, called "Jamie", as a template for the human species. The result was a reanimated Jamie obsessed with finding his mother. When the Empty Child touches another living being, the nanogenes spread like a virus and try to "heal" the recipient, giving the same gas mask and cut hand the child happened to have. Since they are gathered under a hive consciousness, the resulting empty people also adopt the obsession with finding Jamie's mother.
In The Mod Edit
Like the series, empty mobs spread the nanogenes through touch, unlike the show however, the mobs must first kill the victim. They will walk around like zombies with the same melee attack. Their spawn rate or requirements are currently unknown.