Harrek the Berserk is another of the four main driving forces of the Hero Wars. Here’s how he gets introduced in WBRM:
Harrek the Berserk, the White Bear of the title, is mentioned in almost every legend, history, and saga that remains from his time. Chroniclers across the world wrote of the misfortune that this man visited upon their lands. Harrek gained fame as a professional gladiator, a wolf-pirate, a nomad khan, a mercenary for the gods, and as an iron-fisted tyrant of his own realm, who never forgot his common origins and never let a poor man starve. He deserted that realm, and never returned, to come to the aid of his friend Argrath, and to fight the empire he always hated.
Harrek is a combination of Heracles, Rustam, Achilles, Conan, and Harald Hardrada, with a dash of Egil Skallagrimsson for good measure. He is the mightiest warrior in the world and loyal to his (few) friends, but brings destruction and war wherever he goes.
Again, we have plenty of latitude for interpreting Harrek. Was he a murderous madman? A Conan-figure whose adventures bring him to the very heart of Glorantha’s mythology? Was he more of a Rustam figure who brought destruction often despite his best efforts?
I think Rostam is an under appreciated source of inspiration for Harrek.