IMPORTANT - PLEASE READ (or check the sticky comment)
Interesting NPCs have received an update that fixes this issue, so this mod is no longer needed if you run 3DNPC v4.51+.
This fix is safe to uninstall/remove at any time.

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This is a simple fix to the qf_carmellaskjarnquest_032a993f.pex script from 3DNPC.
Please note that this description includes potential spoilers for two amazing quests added by the mod Interesting NPCs.

TL;DR: If you have problems with Skjarn attacking Edwayne during Art and Sacrifice, this fix is for you. Install this mod, load a save before meeting Carmella in the cave, and you should be good.

About the bug

This bug only occurs if you don't let the "duel" scene in Solitude during Bards, Beasts, and Beauties play out to its full extent. It is easy to get the bug, because you have to wait a while for the scene to finish and it is not entierly obvious when its done. What happens here is that Edwayne and Skjarn both have their own faction which references each other and is used to toggle the hostility towards each other through the Aggression actor value. Once Skjarn and Edwayne starts to fight each other in Solitude, the aggression is changed from 0 to 1. If you leave the cell during this scene, the actors might get stuck with this actor value, causing a future quest (Art and Sacrifice) involving these two NPCs to break.

When you enter the cave to meet Carmella druing Art and Sacrifice and reach the NPC in question, Skjarn will appear. If his aggression is bugged and still set to 1, he will immediatly start attacking Edwayne. Edwayne will then proceed to make a run for it. This is not supposed to happend and the quest-scene will break, causing you to be unable to proceed with the quest.

Typical symptoms include:
- Skjarn rushing to bash Edwayne's head in the moment he spawns into the cave where you meet Carmella.
- Edwayne uses his vampyric superstrength to rip off the shackles and proceeds to run out of the cave being chased by Skjarn.
- Carmella stands still with a look of disapproval and the quest fails to proceed.

About the fix

This fix only adds a 2-line scripted safe switch to make sure both actors have the proper aggression set during Art and Sacrifice. This way, you don't have to worry about screwing things up. It does not change the existing script instance in a saved game, so it should be safe to use and install mid-play-through. It replaces the original quest script included in 3DNPC, so I recommend that you use a mod-manager tool like Mod Organizer 2 to install this fix.

Personally, I've spent probably 5-6 hours testing this stuff out on my own 50 hour long save game. Tried different angles to screw up the quest, etc.
If you use this fix and still get an error, let me know!

Make sure you load a save game prior to meeting Carmella in the cave. This is the point where the quest breaks if you have the bug, and the fix does its thing the moment you approach her, so it will not repair the scene if its already broken (mini-duh). It must be played again, so for most success - load save game before entering the cave or just before meeting Carmella inside the cave.

Alternative fix
You can resolve this issue by using the console, if you don't want to install my edited version of the quest script. When I first encountered this bug, I simply pulled up the console, clicked on Skjarn the moment he spawns in the cave and type "removefac XX0B71D6", replace "XX" with the mods load order ID. There are other ways to do this, of course - but this is what I did before I made this fix. You can also click on Edwayne and do "removefac XX0B71D7" as well if the first suggestion doesn't work. It must be done before Edwayne starts running.

Installation

Requires Interesting NPCs SE (3DNPC).

This is just a script replacer, so it does not contain any .esp, .esm or .esl files.
It only replaces a script included with the original 3DNPC download.

Install with a mod manager, preferably Mod Organizer 2.
Activate it in the left pane. (if you use Mod Organizer 2)
Make sure it is placed below Interesting NPCs in the left pane so it actually replaces the original mod's script.

I don't use Vortex, but should work fine there as well, just enable the mod and make a rule for it to overwrite files from 3DNPC.

For manual install (not recommended), simply drop the folder inside your Skyrim Special Edition's data directory and overwrite.

Without the original mod, this fix is useless.

Uninstallation

If you followed my advice to install this mod with a mod manager, simply deactive the mod / right-click and remove mod.
If you installed this mod manually, you'll need to grab the original script again from 3DNPC and replace my script with the original one.

The name of the script-file is qf_carmellaskjarnquest_032a993f.pex

If 3DNPC updates, you should remove this fix until further notice.

Compatiblity

No issues with compatiblity are known.

I use 900+ mods with 254 plugins and it has worked fine for me the 10+ times I've tested this.

It should be easy to spot if there are any other mods which changes this script through Mod Organizer 2. If you see a lightning symbol with a red "-" mark on it, it means it has been overwritten by another mod. This happens if you have my mod loaded before 3DNPC in the left pane. My fix should be loaded after.

Credits

All credits to the excellent kristakahashi and the team for making Interesting NPCs. Thank you for this great mod! I never play without it.
I've uploaded this bugfix here on the basis of what's stated on permissions on the original mod's Nexus page.

"Modification permission: You are allowed to modify my files and release bug fixes or improve on the features so long as you credit me as
the original creator"