This is a special ticketed event, I believe it cost around $38 per adult last year and it was worth every peny. We went to the party twice because we enjoyed it so much. The party offically starts at 7pm and goes until midnight, but you can get in as early as 4pm. They give you a special wrist band and once it gets close to 6pm (that is when the park closes for the night on party nights) they start shuffling all the party goers into adventer land. The rides and some CS and LTT are open.
Some of the rare characters that we saw were the villians. They have a villian meet and greet that we did not attend so I am not sure as to which villians are there. The parade and Hallo-Wishes - the firework show are awsome. We would go at this time again next year but I want to try Disney at Christmas time. If you are thinking about going, I would say defiantly go, it is so worth it and it is just a great time.