It's an add-on. It feels tacked on. Only buy if you totally love Blood 2.

User Rating: 5.8 | Blood II: The Chosen - The Nightmare Levels PC
The Nightmare Levels are an add-on to blood that truly are tacked on. The premise is tacked on and used to extend the story a bit, and the pacing and story are all over the map. The general premise is to explore the major characters in Blood 2, each (4) with a unique story.

As I said, this is really only for those who care. The levels are interesting, but not really notable. The new weapons ARE cool, but sadly aren't put to as much good use as they could be. On top of all that, the add-on is extremely SHORT, with about 4 hours of total play time. Save your $$.