I'd probably just offer a save at -2 or -4 and shorten the duration. It wasn't terrible with the MR check, although it could be with a mage around to take care of the MR.
Yeah, save a la Celestial Fury would be fine, or % - based chance. The problem with this weapon is that at this point in the game, oponnents which we can hit (not under PfMW) usually die so quickly that THAC0 drain is the least of their problems. 
Parthis, 8th BG2 update
Where were we? Ah yes. Planar Sphere. Not much to say. I like Gesen bow a lot.

First major battle - Lavok. Apperantly the only caster thus far to summon Ultroloths. It held my archer with it's gaze, and died.

Satzee did fail a save vs ADHW and died soon after.

Lavok summoned some undead, and that was it from him.

Demon plane - no dangerous gating this time, only Nabassus. At this point, sheer XP harvest.


Archer with WWs craps them in a round. I'm happy. 
Tolgerias was a bit harder. We didn't buff extensivly, just waded in. He does his usual Planetar/Alacrity crap.

WW attacks take care of it.

Next he gates in a Pit Fiend. I was expecting the worst (Balor); Pit Fiend is a joke compared to it. Satzee's poor saves allow her to suffer a Stun symbol.
barbarian is chased by a Mordy, and can't do anything but run.

I focus down Pit Fiend first, he wastes his Wish on himself (yup, it's a selfish demon).

Barbarian finds an opening at kills it.

Parthis removes stun from archer, Tolgerias keeps summoning demons & djinni. However, with his PfMw expired, he's prone to spell failure...

He explodes.

We ramp the last demon easilly and return to home plane.
Now, Bodhi. I was expecting difficulties here. Apperantly Parhtis could blow some lesser vamps with Turn Undead. Cool, didn't expect it.

Fighting them in melee is *always* bad...I let Deva do the ugly work. They seemed to run away from it.

There was one bad endeavor - archer charmed.

barbarian dead...time to go invisible (Parthis' CON is at 8 here)

Buff up, kill the mage..

..and whatever is left.

As per Bodhi...sigh. The plan was to use two Pro Undead scrolls - Parthis and archer, so at least other vampires would ignore them. Turns out - Bodhi can't see through it! 
That's pretty dissapointing. 91K XP enemy, thwarted by a scroll available in any temple? Ah well...so be it.
I hide Burth with invis potion.

We slap the vamps hard, safe under Pro Undead scrolls.



She won't die unless she talks to someone, so I get barbarian there.

I finish it with WWs and return the Lanthorn.

Liches are next.