The over-saturation of zombies these past few years is often remarked upon, and add-ons shoe-horning them into otherwise unrelated games are far from uncommon. But this downloadable side-story (which I bought as a stand-alone disc), depicting the sudden intrusion of a zombie plague into the waning wild west of Red Dead Redemption, is one of those instances where two disparate parts mix to produce a strong, internally consistent tone.
The dry humour and moral ambiguity of the original game lead to an unusually Romero-esque zombie apocalypse, where the dead show glimpses of humanity, pathos and slapstick. In keeping with the God-fearing setting, they also have a decidedly supernatural element, displaying inhuman strength and moaning in creepy, distorted voices. Coupled with a considerable resistance to being shot anywhere other than the head, an encounter with a horde of these zombies carries genuine threat. In every way, these are far from the usual cannon fodder that we've come to expect from the video game undead.
Like the game it branches off from, Undead Nightmare excels at allowing you to inhabit its creepy world, oozing atmosphere and character, but, sadly, it's not really sure what to actually do with the setting. The main storyline involves finding the various characters from the original story and seeing how they've adapted to the dead rising from their graves. This bit is superbly handled, but once that's over we get only a highly unsatisfying ending that is then immediately undone in order to provide the new game+.
This whole new take on the original game is an astounding endeavour even so, but given how the "real" story so adeptly characterised its protagonist over the game and provided a note-perfect resolution, this otherwise entertaining spin-off can't help but suffer in comparison.