Red Dead Redemption is a milestone for open world videogames and was the first ever with a western setting. But Rockstar Games decided to go one step further with their DLC Undead Nightmare, which turns the game into a zombie-apocalypse-western.
Rockstar's story and mission design are excellent as always. Their games' biggest strength is the satirisation of the United States of America and it works here again. Their developers' creativity has no limits, the player profits immensely from that.
This time there are bigfoots, burning horses and an unicorn.
The only aspect you may criticise is, that it is too short. There are only 10 story missions and some of them are pretty easy and a bit boring. So that's why the main game is superior to this dlc.
However both of them are recommended to play. These are historical pieces of art you can learn from, and most importantly, that will give you a lot of fun! Before watching a zombie movie, just play the best one available!