This is a basic summary of the ball scene and the events afterwards. Some minor points may be missing, but I think I got everything.
The White Council received an invitation from a new noble of the Red Court (Bianca) and Harry was the closest representative so he was the one obligated to turn up. He could have refused, losing a bit of face for the White Council at the same time, but circumstances with the spirit torture spell forced him to. So when Harry and Michael turned up to the ball they were included in the Laws of Hospitality as well as under the Accords (one rule of which is that you cannot specifically target a person at an Accords meeting, which is why Bianca had all of the drinks spiked rather than just Harry's). Any hostile action taken against Harry or his plus one would have broken the Accords and allowed the White Council to seek redress.
Unfortunately, Susan had found the invitation and had a copy made, which allowed her to enter the ball without any kind of law or tradition protecting her. Since she was effectively an intruder it made her fair play for all the assorted monsters at the ball. Harry naturally rebelled against this and tried to protect her, which would have constituted taking a hostile action against the hosts of the ball, thus violating the Accords (Bianca could have demanded his head in recompense for this).
Thomas then decided that he wanted to leave the party alive and threw Susan into the throng. Harry reached for fire and fire answered, possibly casting his Death Curse at the same time, got revived by Michael and then saved from smoke inhalation by the Leanansidhe. They headed back to Harry's place with Thomas turning up a time later with Amoracchius.
Harry + Co decide to rescue Susan through the Nevernever, with Harry having to dodge Lea by eating some kind of deadly toadstool. He gets a year and a day of freedom before Lea can try anything against him again. Things go wrong and Harry ends up captured by Bianca and left in a room with Justine + the newly turned Susan. He couldn't be eaten by the other vampires because of the mushroom poisoning, but Susan is about ready to make her first feed and become a true Red Court Vampire.
The Nightmare strikes again, possessing Justine just to tell them that he's waiting for Harry to fall asleep. Harry concocts a plan to let him defeat the Nightmare using his own ghost and eats the Nightmare in his dreamscape, absorbing its power and the power it stole from him earlier in the novel. Susan or Justine give him CPR to restart his heart. He then retrieves his stuff from a nearby room (Bob included) and confronts Duke Ortega, Bianca, a bunch of vampires and some mortal gunmen. Bianca offers to let this all blow over if she can keep Susan as a form of revenge for making Bianca too weak to avoid killing her mortal plaything back in Storm Front.
Harry takes issue with this idea. War is declared and Harry gives a bunch of power to the hundreds of ghosts. They kill Bianca, cause the building to implode and Harry escapes with Justine, Bianca and Bob.