What can I expect from a halloween party?

So this is a bit embarrasing but here it goes. I'm not looking for judgement, I'm looking for advice. I am a 24 year old grad student, I can't drink because of a liver problem so I generally avoid parties except with my closest friends because I spend the whole night turning down drinks. I will admit that I am somewhat antisocial. I have been to family parties and events but never just a straight house party. I am visiting my fiance this weekend and he wants to go to his friends halloween party and I want to know what to expect. Judging from the evite about 20 people were invited. They are mosty law students and I only know a few of them in ping. Should I expect dancing? or mostly just chatting? games? what? I am really fine with the whole schmoozing thing but I just don't know what to expect. I know by this age I should have had this experience already but it is what it is. Also my halloween costume is Carmen Sandiego. I have made it y within reason with some black tights and thigh high boots with a low neckline. Thoughts?