This single player DLC is completely separate from the inFAMOUS 2 story line. If you do not own inFAMOUS 2 (which I don't understand way anyone wouldn't), you can still download and play Festival of Blood without being lost as to what's going on in the game or needing the inFAMOUS 2 disc to play. There's also a quick tutorial of the controls for all you first time players.
Now, I bet you're thinking to yourself right now "How could the inFAMOUS games possibly get any better?" These games already come complete with a hero you can make either good or evil and has some pretty great powers. When you get into these games it's hard to rip yourself away from them, but what was missing? Vampires.
In the beginning of the game, Cole goes to the catacombs of the church in New Marais to help the people that got trapped by a collapsing ceiling. He goes on saving one person to the next until he gets to his last target who then attacks him! When he comes to, he sees that his blood is going to be responsible for bringing Bloody Mary, the head Vampire, back to life. To show her gratitude towards Cole she decides to give him the gift of immortality. From that point on, you learn that by daylight she will control Cole and make him one of her minions. You have 8 hours (no, there isn't a timer) to unravel the mystery of defeating Bloody Mary. If you do not make this deadline then you will be under her control forever.
Those who are familiar with inFAMOUS 2 may be happy to see that the UGC (User Generated Content) feature is back with some Festival of Blood missions created by fellow gamers. Festival of Blood adds some powers to Cole's arsenal that compliment his existing powers quite nicely. The more you dig into the game, the more powers become available to you.
If you're a fan of the inFAMOUS games but have been on the fence about this DLC, I would say definitely download this. It keeps things close enough to the disc games where fans will enjoy it and also be happily surprised by the new powers.