Extinction Party

From the introduction: A group of friends gets together every year for a themed Halloween Party, and at Midnight everybody takes of their masks at the same time for a “grand revealing” and a fireworks display. This year’s theme is “Extinction Party!”, and guests are expected to arrive as dinosaurs. However, last year alien spores contaminated the punch with symbiotic aliens that are now riding along their hosts. At the end of this year’s party they will decide whether they have learned all they wish to about humanity. If so, they will leave the planet during the fireworks display, and bomb it from orbit.