ButtGoat

But it sounds like this is a problem with an object on a creature. Especially if your save game works in other areas but not this one. The only foolproof way to troubleshoot this is to CLUAConsole:CreateCreature("cre") for every .cre in the area file, and possibly those in the .bcs too. And you can find those out by looking at the files with an editor.

That's what I was thinking. I'll see if I can get that done at some point. I'll post results when :-)

Could you try installing this creature fixer?? (I hear these kinds of errors pop up when you have a creature with a 2-handed weapon equipped with a shield The fixer fixes that )

Done and done. But it didn't fix my assertion problem :-(
Still the same error. Should I try removing the weapons on my avatars? (Didn't work)

The only was this wouldn't fire is if he's not in ar0907.are

He's not (the OP is still downstairs).

He shouldn't ever "stays to fight me," though (and he's appropriately invincible because of it). Both him and Ketta are intended to leave after the dialogue.

I guessed as much :-) I'll just leave him be then. Out of sight, out of mind.
Thanks all!

@Setharnas
Thanks for explaining :-)

@Kitanna
Awesome! Thank you.

I'm still looking for some help to fix for invincible Sion though.
And I am also experiencing a "has nothing to say to you" bug with Guardian Telwyn in the Temple of Helm AR0902, as discussed in this thread. As far as I could see, no solution was ever offered, as setting the globals did not work.

EDIT: Even though I originally got the quest from the Helm priest, I completed it with the Lathander followers. I suspect one of my mods like no stronghold restrictions were causing the problem (my alignment is NG). Sorry for bothering you guys :-)

Here are changelogs:

Mods affecting AR0901.BCS:
00000: /* from game biffs */ ~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ 0 0 // BG2 Fixpack - Core Fixes
00001: ~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ 0 3 // BETA Core Fixes (please check the readme!)
00002: ~TELEPORT/TELEPORT.TP2~ 1 0 // Teleport spell for BG2:SoA
00003: ~BG2_TWEAKS/SETUP-BG2_TWEAKS.TP2~ 0 1160 // No Restrictions (Baldurdash)

Mods affecting AR0901.ARE:
00000: /* from game biffs */ ~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ 0 0 // BG2 Fixpack - Core Fixes

Mods affecting SCTELWYN.CRE:
00000: /* from game biffs */ ~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ 0 113 // Additional Alignment Fixes

Mods affecting SCTELWYN.DLG:
00000: /* from game biffs */ ~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ 0 0 // BG2 Fixpack - Core Fixes

I have no idea what to do about this.

Ok, here are the changelogs:

AR0204
Mods affecting AR0204.ARE:
00000: /* from game biffs */ ~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ 0 0 // BG2 Fixpack - Core Fixes

Mods affecting AR0204.BCS:
00000: /* from game biffs */ ~TELEPORT/TELEPORT.TP2~ 1 0 // Teleport spell for BG2:SoA
00001: ~TELEPORT/TELEPORT.TP2~ 1 1 // Teleport spell for BG2:ToB
00002: ~SETUP-NEH'TANIEL.TP2~ 0 0 // Neh'taniel NPC Mod

*bangs head into wall* - Changelogging is a nifty thing to be acquainted with. I uninstalled the Neh'thaniel mod, problem solved. Though I would have like the mod to work; do you have any explanation as to why it doesn't? :-)

Sion
Mods affecting HLSION.CRE:
00000: /* from game biffs */ ~BG2FIXPACK/SETUP-BG2FIXPACK.TP2~ 0 0 // BG2 Fixpack - Core Fixes
00001: ~SETUP-TACTICS.TP2~ 0 6 // Improved Guarded Compound in the Temple District
00002: ~BG2_TWEAKS/SETUP-BG2_TWEAKS.TP2~ 0 3011 // For Non-Party-Joinable NPCs Only

Mods affecting HLSION.BCS:
00000: /* from game biffs */ ~SETUP-TACTICS.TP2~ 0 6 // Improved Guarded Compound in the Temple District

I've attach the .BAF file
I would try and fix the minhp, but I don't know what it's supposed to say, need help :-)