The Chief's Daughter

The settlement of Dun is a safe place, fortified from raider attacks, with its own spring to provide fresh water for all. The spring has never failed them - as long as it flows, Dun will never fall. When the village captures a young raider, Nessan pleads to keep him alive, against the wishes of the elders. As the Chief's daughter, she wins out. Their captive settles into the village, he and Nessan become cautious friends. Things are peaceful. Until the spring begins to dry up. Have they angered the gods by not killing this raider? Or is there some other force at work?