For those who don't know, Surtr is one of the Jotunn, a race of giants with powers that rival the gods themselves. Surtr will lead "Musphell's sons" to Ragnarok, which I now assume could be an army of Surtlings and other fire creatures who natively live in Musphelheim.
Surtr himself could likely not be the final boss, as he guards the entrance of Musphelheim.
Musphelheim is the realm of fire, perhaps the Ashlands were created when one of his sons was lost or sent hurtling through the realms as a comet, and crashed through the surface of Valheim, tumbling into the earth and being trapped forever, imbuing the centre of Valheim with immense heat, which shoots up through the surface alongside small fragments of himself that manifest when droplets of the magma he is merged with are splashed on surfaces, and become his children. Maybe the massive hole he once carved into the earth is still there?
Surtr is well known for his massive flaming sword, which is honestly incredibly badass and sounds like it would look amazing. I was thinking about how big his son would be, how the encounter would play out etc, then realised something amazing.
The lord of the rings, has the famous demon of the ancient world known as the "Balrog", which even uses a flaming sword and whip! I realised upon watching the clip that it was exactly what we needed to see in this game.
Now watch this, and imagine it being in Valheim, as a lost son of Surtr, who years of isolation has turned him into a ravenous monster :