Undead Nightmare is possibly the best expansion pack of all time. If you own Red Dead Redemption you need this.
This is possibly the best $10 I've ever spent. You play as John Marston in an alternate storyline where the Zombie Apocalypse has spread throughout the land. Marston's wife and son fall victim to this plague and Marston sets out to find a cure. You'll soon be on your way from town to town saving other survivors and finding ammunition. Ammunition in this game is basically currency, since money is useless with Zombies ravaging the land. You have to play this game smart for the most part. This means being conservative with ammo, and making sure you aim for the head. You meet characters from Red Dead Redemption like Wes Dickens and Seth, and of course you have to do missions for them.
Graphically this game is the same as Red Dead Redemption, as you would expect. The zombies are diverse and make you miserable when there are so many around you closing in. There are also mutated zombies, some who seem to have been borrowed from Left 4 Dead, such as a "recher" which spits a glowing green goo at you, the big guys who charge at you and even little monkey like zombies who move fast and are hunched over, making it harder to hit them in the head without using Dead Eye. Even animals are zombies in this game. Zombie Bears, Zombie Cougars, Zombie Wolves etc. Its very cool.
You ride from town to town eliminating or lowering the zombie threat. Save one town and move to the next but be warned, the Zombies will be back eventually and you'll have to defend that town again. You can purify the land by cleansing cemeteries which is basically a horde mode. As you burn coffins more zombies come out, burn all the coffins and eliminate all zombies and the land will be clear. There are also rescue missions where you can save missing people.
This game also features unlocking the Four Horses of the Apocalypse: Death, Famine, Pestilence and War, as well as an appearance by Sasquatch. The DLC pack also has new multiplayer modes which I haven't really played so I feel I cant speak on them.
All in all this is a masterpiece that is even worth the $30 price tag of the disc based version, this is much more than a cheap DLC pack to get your money, it is very entertaining, well thought out and even exceeds its $10 value.
A must have for Red Dead fans.