It's about, like most Ensiferum songs, living an honorable and glorious life, of course, the actual information is a little bit outdated, since modern day Norsemen don't raid Europe anymore, and mostly Christian, but anyways, as someone explained, the dark steam signifies death, and after death, only true heroes who have lived gloriously will be strong enough to cross the river into a heroes after life. Those too weak will "drown" in the stream, that most likely means they get swept away and taken to another afterlife. The eternal heathen throne is this heroes afterlife that Norse heroes strive their whole lives to be a part of, where their souls will be at home, they are most likely referring to Valhalla.