Alan here sharing love and wisdom.
I can't tell you where your weapon is but i can give you some possible theories as to what happened to it. Given my experience testing and messing with the bioware scripts and aurora toolset i have played a few persistent worlds who altered the disarm script. The original concept is that the disarm feat would cause you drop the weapon where you last stood this often became a problem for players losing items in a fight. some times due to broken tilesets, poor placables, or otherwise glitch glitch nwn core scripts some times the item would randomly vanish. This was fixed by Hordes of the Underdark but still was known issue to happen very rarely.
Introducing disarm scripts and custom hak disarm scripts. With changes to the disarm script and to keep those pesky adventurers stealing npc weapons and modified plot items some people designed a script to make the disarm object appear in the inventory. Which was a good idea some also made a cool script where a successful or overly achieved critical disarm would make the person who disarmed you acquire your weapon almost like a pick pocket skill. witht aht said this is what possibly happened to your weapon.
If the disarm script was the original concept and you were disarmed and it fell to the ground two things could have happen. 1> you were disarmed and given if a custom/non script when disarmed moved said weapon x amount away from you it may have fallen away from your battle if there were no appropriate tile sets for the weapon to land on it a> disappeared or B> fell on a tile that was suitable but not in distance where you could have seen it. Given the custom tiles of the mountain this is more likely what happened because the smooth terrain tile sets doesn't calculate the ground placement as easily as old school nwn flat grounds because of the variables.
2> This is the most definite answer I can give that happen to your weapon. It disarmed you with a 20 and i'm not sure how the customs scripts for disarm work but if its like some places i know that thing disarmed you. On a literally 20 its an auto yoink from your hands he disarmed you Bruce Lee style. The creature claims the weapon and takes it into their inventory which most scripts will either arm them with your weapon or it will be in his inventory. Now here's the kicker if that enemy does not have the proficiency to wield the weapon or has a creature item list only.... it done ate your weapon girl! Nwn with target creatures who have the creature item functionality automatically drop the weapon cause they can't use it that's how the script works. But because the script means they steal it from you on a crit and cannot retain it in their inventory and the item cannot be drop it glitches and becomes invisible in the inventory. Or how i like to call it "Script Eaten" the problem with that case is if your lucky and depending on the treasure model of the creature when you kill it it's corpse or drop remains should have your sword in it for you to reclaim this usually is the case with this scenario. However if the creature genuinely has these features more or less the script ate your weapon cause it doesn't have a place to put it. Given these factors it could be either of these or all together meaning the area or the monsters may need to be rethought to avoid this issue.
