Offline
I have a question about the magic item, Horn of Valhalla (pg. 477). Are these wild Berserkers magical beings like spirits, like those old ghosts who fight for Aragorn in Return of the King? Or are they actual men who are somehow magically just dropped in the middle of wherever the horn is blown, and immediately content fighting for their caller?
I know that the ones called in the Runegraver ability are of course real men from their description. But the Horn of Valhalla seems to work immediately, and the mention of the berserkers “fading” after combat further leads me to believe them to truly be arriving from Valhalla.
Offline
I think it's best to leave it open-ended for you to decide. Perhaps they are spirits drawn from the Hall of Ymir, who fight on your behalf until such time as they fade away. Or perhaps they are pulled from their lives elsewhere in Hyperborea, causing some great calamity on the other side of the world. Maybe a dozen or more of the finest wild berserkers never return to their home, causing a ripple effect in which an entire tribe or village is disrupted or overtaken. Maybe survivors speak prayers to the gods and are answered -- they find out who this cruel being was who blew this horn and ultimately effected the ruination of their lives. They swear to exact vengeance!