I have speculated a bit on this, trying to tie it to historical events.
The icewind dale games hint at battles with fiends and thrunn seems to fit the bill as a bestial like demon.
His binding by druids and the stones predating the ten towns also fit so I had thruun be a survivor of the battle between reghedmen and an archmage of ythryn. When jerrod closed the portal the shaman couldnt kill thruun as he kept regenerating so fast. So they bound him beneath the stones.
It's a work in progress on my wordpress site if you want complete homebrew based on canon and semi canon (computer game) events
I've had a quick scan over what i did previously and i think the reghedmen that formed around -2100 DR had a big fight with the last remnants of Ostoria around Kelvin's Cairn (led by a giant whose name corrupts into Kelvin).
The reghedmen then claim the mountain and the surrounding hotspots for their prime migrating route, its warmer there and good grazing land for longer than the rest of the tundra.
Then the Fall of Netheril happens. One of the arcanists (arakon) escapes Ythryn and gathers the remaining giants and orcs and goblinoids into an army and tries to take Kelvin's Cairn. He is searching for stuff from Ostoria (which is what Ythryn was looking for in Icewind Dale in the first place).
This "shaman" (the reghedmen have no idea what an arcanist is after 2000 years of barbarism) and his forces were attacked by the Reghedmen during their annual migration (because he was on their sacred land). The Reghedmen band together and start to win so Arakon opens a gate and brings through a ton of demons which slaughter men and goblinoid and giant alike.
Jerrod (title of Raag for overking of the Reghedmen) closes the gate by sacrificing himself. THe Reghedmen win the day and the demons are slain, bound, or flee. Thruun is one of those too powerful to slay so 20 Reghedmen shaman bind him to the ground cutting off their own hands as a sacrifice.
The Reghedmen hereafter regard the mountain and its land as cursed and so avoid it, but will not permit anyone else to settle here (which is why they attack the Ten Towners).
Thats my version of it anyway, weaving bits from various sourcebooks and the computer games and the new adventure path.