"It's the best bow in the whole game. Yes, it's an "artifact". It's not a quest item. It has repair difficulty level 30 so that you will need someone with Repair Item 10M or more if you want to repair it. Blazen Stormlance is 10G in Repair Items, therefore, free him ASAP, if you haven't done it already He can repair the bow. The behaviour of the shops with respect to the bow is a bug in the game. You cannot sell it. You cannot repair it in a shop."

But IMO it's always been a bit odd. I've found that the bows "of Carnage" (or with "explosive impact") behave very oddly in other games, too. The artefact bow Perceval in MM6 sometime does only 1 or 2 damage, even though you can see the explosion when it hits.