I've finally thought a new bit of food for thought! :D
The Avisarr is Draxorith's Soul Gem. It holds his soul and gives him a way to exist here on the Prime Material Plane. Most of the information about it is covered under the Avisarr Quest, so there's not a whole lot to ask about it.
However, I was wondering. What would happen if someone succeeded in destroying it. Not throwing it into the banishing chamber with the rest of his essence, or killing him and destroying it, but simply destroyed the gem and he was still in the banishing chamber. Would he survive? Would he become soulless? If so, what would that mean? Would he immediately die, as opposed to being banished or unable to appear on this plane?
Obviously, it would take someone, probably an archmage, of immense power to destroy the gem, but it just made me wonder.
Me personally... if I were the DM of such a game and some ridiculously powerful mage actually managed to destroy the Avisarr? Well, first of all, I would make it REALLY hard to destroy the Avisarr. I mean, that's the once in a generation kind of kingdom saving feat that the bards sing about. Like destroying the One True Ring in Middle Earth. It would be the conclusion to a grand adventure.
If the Avisarr is destroyed, Drax becomes mortal (and hence, killable). As long as the Avisarr exists, he's essentially unkillable and immortal. You can destroy his physical body on the prime material plane (i.e. Khoras, Greyhawke, etc) but he simply reforms in the depths of Hell/Abyss/other naughty places and could eventually return to the prime. If you want to kill him permanently, you would have to destroy the Avisarr first. Then, if you kill him, he's gone for good.
At least, that's how I would run it. Keep in mind that, if you destroyed the Avisarr and killed Draxorith, you would also be responsible for wiping out a major cult and shifting the political balance in eastern Ithria. :)
Ok, cool. Of course, Duthelm losing it's status as the enemy of all of eastern Ithria WOULD be a rather entertaining prospect. Of course, I'm pretty sure it would never happen. Besides, if anyone ever ran the Avisarr, there would be no point and no opportunity anyways, as most groups would do the easiest, most logical thing and just throw it in with the rest of him. But it was an interesting idea.