Living corpses who find their origin in West Indian, Caribbean, and Voodoo legend, zombies are used by evildoers as servants.

The zombie is actually a poor soul that, having just been buried, in unable to move peacefully into the afterlife because it is called forth by someone on earth.
Once it answers, the zombie climbs out of its fleshly dug grave to heed its master's call, obeying whatever commands the master makes.

With human nature being what it is, these orders are usually wicked.

To ensure that their loved ones get safely into the afterlife, many West Indian cultures bury their dead face down, with their mouths full of dirt and their lips shut.