I never got the Nightmare pack,I needed money for another game and so I traded in the game

,I really miss it But instead of re-buying it and passing it all over again(I deleted my saves

)I'm thinking of getting or renting the Nightmare pack But I have a few questions!!!......
-Is the disc separate from the regular RDR?Such as GTA IV's DLC packs sold as games on there own with the original GTA IV not being needed?
-How long is the story?
-Is it all DLC packs or just the zombie one??
-Is the online modes from the regular RDR or all new stuff??,IF like the regular then could I play against people playing regular RDR or just the same who have the pack like with TBOGT for GTA IV?