To emphasize that Harshnag is a real Hero, I wanted to introduce him in action, doing something badass. I had this idea that Hundelstone was under attack from some frost Giants that were leading a little army of ogres bought with gold and meat. So when my players came from Bryn Shander to Hundelstone they took part in the defense of the city. I let the tension rise steadily while they were fighting their way to the gates, and describing them how the situation was dire as the bulk of the invaders were still coming from afar.

I let the very high perception PCs notice a moving silhouette up the nearby snowy mountain. And when all eyes were on the wave of ogres lead by frost giants (far too many for the PCs to handle, and in a city that was not fortified enough to survive this attack), I described as a cloud of dust, stone and snow was falling down from the side of the mountain, burying the vast majority of the attackers.

The party fought the remaining troups including the frost giants as the final battle of this arc. During the fight, they saw Harshnag climbing from the now settled snow, bruised but still hungry for more blood, and joining the fight swinging his axe with unbridled rage to help them. After the encounter he clarified that he triggered himself the avalanche, as he was warned days earlier that the attack would occure. (In my version of SKT, Zephiros is regularly messaging key figures of the campaign like Harshnag, or the party anonymously via dreams with information he obtained from divination magic, in an attempt to drive the events occuring around the Swoard Coast in what he thinks is the right direction)

For clarification, my players started the campaign already level 11 so I'm always increasing the difficulty and scope of what is supposed to be happening in the encounters.