Malevolent sorcerer and costume shop owner Ethan Rayne (Robin Sachs)
casts a spell that turns the children of Sunnydale into their Halloween costumes
whereupon children wearing ghoul masks become actual ghouls
and attack kindly old ladies.
This is remarkably similar to Conal Cochran's (Dan O'Herlihy) plan in HALLOWEEN III to turn children into piles of killer snakes and spiders via his Stonehenge-powered mask factory.
In BUFFY, this also means that Xander (Nicholas Brendon) becomes a soldier,
and Buffy (Sarah Michelle Gellar) becomes a helpless duchess from the 1700s.
This is all pretty absurd, but no more absurd than the glory that is HALLOWEEN III: SEASON OF THE WITCH. Happy Halloween, everybody!
2015 HALLOWEEN COUNTDOWN