It’s the spring of 2018, and the city of Austin gears up for the annual South by Southwest Festival, which draws tens of thousands of people from across the world for a few weeks of film, music, tech and more. While the venues rush to prepare the stages and the city flings open its doors, one young man prepares to make his own headlines. A devastating package bomb explodes at a home in the Austin suburbs, and law enforcement officials think it’s an isolated incident. Within days, the Austin community and the entire nation find there is plenty to worry about as a trail of destruction rips through the city. Following the first season of the true-crime podcast “The Orange Tree,” season two of “Darkness” tells the story of Austin’s serial bomber.