Beowulf is an ancient Earth epic poem about a fictional hero by the name of Beowulf. Set in 6th century Denmark, the poem describes Beowulf's fight against the water monster, Grendel, his victory over a dragon, and ultimately his death and funeral.
In 2371, Harry Kim participated in a Beowulf holodeck program where he assumed the title role. While he was playing the program, the USS Voyager was invaded by photonic lifeforms from a nearby protostar; these beings became lodged in the holodeck and caused three crew members, including Kim, to disappear. Voyager's Doctor was sent into the program (for his first away mission) to recover the missing people. (VOY: "Heroes and Demons")