The scene was a stark — and a slightly spine-chilling — sign of the changes that have torn through Saudi Arabia since Crown Prince Mohammed bin Salman, now heir to the throne and prime minister, began rising to power in 2015 and started doing away with social restrictions one by one. And the cousins, along with thousands of other 20-somethings in Riyadh who had rushed to get to the city’s costume shops before they sold out, were thrilled by the chance to frighten each other.