Are hero myths purely based on fiction?

While many hero myths are based on fiction, a great number can be grounded in actual history. Mythical heroes can be found in almost every culture from Hercules (Greece) to John Henry (America). However, many cultures also draw on history to develop their heroes. For example, the centrality of King Arthur to British culture finds its basis in history (a defense of Britain against the Saxon invasion in the 6th Century AD), however the existence of King Arthur as an actual individual is debated. Nevertheless, his persona has become closely intertwined with British culture in a mythical-historical way.