A group of people were having a birthday party at a campsite in Visbek, Lower Saxony, when another group of men armed with baseball bats and torn-off fence posts approached them in the early hours of Wednesday.
A fight between the groups broke out, leaving two assailants gravely injured, one of whom died later in the afternoon in hospital. An autopsy of the 34-year-old man will clarify the cause of death.
Four other men were seriously injured.
Firefighters and police used search dogs and a helicopter to search the area for further injured people, and two men were arrested.
Police said it is still not clear why the violence broke out and believe that some of the participants fled the scene.