The Borderlands 2 Season Pass includes four all-new add-on campaigns featuring new areas to explore, new adventures to undertake, new enemies to overcome and the Ultimate Vault Hunter Mode Upgrade Pack in a bundle that's available at a discounted rate over buying each add-on individually.
The Borderlands 2 Season Pass does not include the Mechromancer, Creature Slaughter Dome or Collector's Edition content as these items were initially available as rewards for pre-ordering and / or purchasing a certain edition of Borderlands 2 and we don't want those fans who already obtained this content to pay for something they already have.
For those who did not receive the Mechromancer, Creature Slaughter Dome or Collector's Edition content with their purchase of Borderlands 2, these items are now available as premium downloadable content.