What Makes Bolts of Kessak the Devourer so special?

With a speed of 5, they're also substantially faster than any other bolts in the game, all of which are rated at a speed of 10.

Other than that, that's about it.

I see it now.

In addition to being fast, only Kessak the Devourer bolts are +2 magical. Weapons that are less than +2 magical are completely ineffective against the fiend in Curst in Carceri (AKA the Fiend from Moridor's Box, all grown up). Nordom never has access to any other weapon that can touch him or the big bad fiend that can be occasionally be found in UnderSigil towards the end game. Morte's teeth are useless against these guys too.

The totally optional UnderSigil fiend drops perhaps the best magic items in the game (besides the Modron Cube and the Portal Lens).