Halloween costume transformation

It was a night on Halloween, and an odd one at that. While you're on your way to a party that you heard would be crazy and would suck to miss- if so, you'd probably be the laughing stock of the whole high school, you've noticed that there's a weird green mist in the neighborhood. You ask a passerby about it, and they say it "must've been an over-dose of fog juice in one of those fancy colored fog machines at somebody's house somewhere". You shrug it off, and head to the place.

The probably self-proclaimed popular boy's house was huge, and decked-out in fake skeletons, spiderwebs and some animatronics assumably purchased from the nearest Spirit Halloween. Anywho, you make your way in after the popular boy himself, standing at the door, let you in. People were dancing, drinking, making out in the corner, spooky music was blaring- y'know, the typical wild Halloween party from people your age.

Looking around for a bit, you hear a weird sound. Actually, everyone heard it, too. It sounded odd, like nothing you've heard before. But it sounded similar to something of a blue whale's cry- an echoy sound that rang through all of your ears. Suddenly and unexpectedly, green flashes fill the room as all the attendees turned into their costumes, and you were certainly no exception.

((Describe your costume. Also, describe your gender and other attributes if you don't plug in a persona by now.))