The "mostly single-player" DLC adds a new campaign "that follows John Marston as he tries to find a cure for the highly infectious zombie plague that has spread across the frontier," complete with new random events, animals, weapons and a location. On top of that, there'll also be eight new zombie multiplayer characters.
Undead Nightmare is slated for release on PlayStation 3 and Xbox 360 this fall, with a specific release date yet to be announced. It'll cost $10 (800 Microsoft Points).
Rockstar promises to summon a teaser trailer for the DLC tomorrow.