Darkon

From the True Films review:

This is a great film about very ordinary people trying to break out of their humdrum lives. The subjects are the type of people who once seem utterly odd, but now ... less so. They are the folks who dress up in historical costumes and role play in fantasy worlds. They are members of Darkon, a live-action role-playing game that's been continuing for 25 years. Every two weekends hundreds of Darkon enthusiasts take over parks in the DC area and stage physical battles for imaginary cells on a paper map.