"Night of the Living Ghouls" is a soon to be released free total-world mod for Fallout: New Vegas. It will NOT be compatible with previous saves, but can be switched on or off at any given time, without disrupting any save data.
The game will take place in Louisville, Kentucky, paying tribute to the city's unique and expansive "zombie culture". Louisville's "zombie culture" includes http://www.dangerrun.com/Louisville-Zombie-Attack/, the setting of the http://en.wikipedia.org/wiki/Return_of_the_living_dead series, http://www.louisville.com/content/zombie-prom-ear-x-tacy-holds-free-family-friendly-zombie-event-halloween, and the hometown of http://en.wikipedia.org/wiki/Robert_Kirkman#Personal_life (writer of http://en.wikipedia.org/wiki/The_Walking_Dead_(TV_series) and http://en.wikipedia.org/wiki/Marvel_Zombies).
Essentially, the city of Louisville has been flooded by radiation in the water flowing down the Ohio River, which caused the vast majority of survivors to become feral ghouls (the equivalent of zombies in the Fallout world).
A number of real world regions locations around Louisville will be recognized by anyone familiar with the Louisville area.