Khaiber Akbarzada is on the verge of being hailed as Afghanistan’s best player of Buzkashi, the ancient national sport where horsemen battle for control of a headless goat. To many, the violence and opportunism on display in the arena are a metaphor for the country’s anything-goes politics — and the line between war and sport is starting to blur. ”Riders on the Storm” captures a pivotal moment in Afghan history and offers a rare and visceral look at a ruthless sports culture where champions become marked men.