I managed to fix this quest by fiddling with the save game file. I used Norbyte's awesome ExportTool from this thread:
http://larian.com/forums/ubbthreads.php?ubb=showflat&Number=621131&page=9I have limited experience with game modding and this is the first time I attempted something like this. I haven't modded Divinity in the past, but I have some programming and game developer knowledge.
This goes without saying but tampering with the save files like this can easily destroy them, and you should make BACKUPS.
Another thing to note is that I am playing Lone Wolf with my wife, so we have no companions. I have no clue how the quest log behaves when playing with companions. But I suspect it is very similar. The changes I had to make within the save file were for both Red Prince's journal(me) and Beast's journal(her).
Here's what I did:
After extracting the .lsv save file using the second tab of the ExportTool you need to convert the globals.lsf to a readable format. You can do this by using the ExportTool's 3rd tab to yield the globals.lsx which is an aprox. 2 mil line xml formatted text.
I found out the quest's string name to be "RC_GY_Godslayer" and it has multiple states associated to it. Luckily I had a save game from before messing up the quest (I didn't want to use it because I would've lost like 10 hours of gameplay). I went in this old save game and basically completed the quest in the safest way possible and saved the game right after picking up the second artefact and having the quest in "completed/turn in mode" so I can get a model of what I am supposed to shoot for, because I had no way of knowing from my present save file the exact string name of what the game looks for when this quest is ready for turn in. Sure enough I found it:
This is how the quest looked for me when it was bugged:
https://cdn.discordapp.com/attachments/312180921501614080/362125346453323777/unknown.pngThis is how the quest should look like (from the older save file). In this state the quest is in "turn in" mode (meaning you only need to speak with Tarquin so he takes the parts, gives you the exp and puts the quest on closed):
https://cdn.discordapp.com/attachments/312180921501614080/362125271505043456/unknown.pngAs you can see, the first image shows the quest completely screwed up because the last state says that I found the Archives Location AFTER finding the 2 parts: shaft(graveyard tomb item) and shards(bloodmyst isle item).
The second image however, shows how the quest should correctly look when it is ready for turn in. Notice how value="HaveBothParts" isn't even present in the first image. I suspected that this is a very important quest state that triggers Tarquin to want to receive the artefact parts. A quick search within the files revealed my suspicion to be true, as the bugged save file had no mention of "HaveBothParts" while the good save file had 3 mentions of it: one for Red Prince's journal, one for Beast's journal and one for identifier tables (more on this later).
Now, the first thing I did was to identify our journals and change the quest state order to look like in the second image -pretty easy copy&paste (I only did this for 2 characters as I am playing Lone Wolf. I am pretty sure, you need to do this 4 times if you have a full party).
This is how the journals look, below each of them are all our quests and all their states:
https://imgur.com/a/DvV2sAnd now for the second part - the identifier tables. These seem to be the game's way of counting and sorting events that happen within the game. This is how it looks in the old save file (the one with the quest completed and ready for turn in):
https://imgur.com/a/cxaQGNotice that MapValue of 7483. Well that is the unique ID that the game associates with this particular event (in this instance having both parts collected). After a quick search I found this value to be referenced 14 times in total within the file (
https://imgur.com/a/sy0XX).
Now, one of the places where this ID is referenced is OsirisVariableHelper. From what I gathered; this is where the game assigns a new ID to its latest event:
https://imgur.com/a/FJHg7It all made sense - I had saved just after completing quest and game had associated the MapValue of 7483 to the "HaveBothParts" quest state. The game seems to create these new IDs by incrementing OsirisVariableHelper +1. I tried to disprove this hypothesis, but I couldn't. There might be more to it, but in my save file any number below 7483 was taken as an ID, while numbers over 7483 were not.
What this meant was that I had to artificially create a new event ID in my bugged save file that would fool the game into thinking that I had just collected both parts of the item and that the quest is ready for turn in. I could not use 7483, because it had been already associated, in my bugged save file, with something else. I looked at OsirisVariableHelper and it was set at 8958 (which made sense because this save file had about 10 hours of playtime more than the other one. I checked to see whether 8959 was taken and it wasn't (which made sense if the game increments IDs consecutively). You will have to check your OsirisVariableHelper and increment your ID correctly.
Now, the issue was that the value of 7483 (HaveBothParts) was mentioned in 14 places in the good save file, which meant I had to add 8959 into 14 places in my bugged save file. This would've been an easy job if the file was shorter and I could get my bearings within it faster. But since the file is a gargantuan 2 and a half million lines long, it was a real challenge to scroll through it and find the exact locations where I had to insert my new ID. Because my bugged save file was 10 hours apart from my good save file, it had grown considerably, and it was hard to keep track of where exactly I need to place my newly created ID.
So my next idea to ease this up was to create a reference so I could better see where to place this value. To do this I hopped back in game on my bugged save file and I did the only sensible thing I could think of to produce a new event ID. I killed Tarquin. The quest log updated saying "Tarquin is dead, we will never know the secret of Anathema" or something like that. This was great, because it served as a reference to where I needed to add the lines from the good save file into my save file. I immediately saved at that point and extracted this save file as well to look in it. Well lo and behold I was in luck, a new event for Tarquin's death was created and OsirisVariableHelper had incremented to a new ID and this ID, which corresponded to Tarquin's death was mentioned 14 times - just like HaveBothParts was!
The next part was easy. I simply wrote down the line numbers from the save file where I killed Tarquin and used them as a reference point back in my save file that I needed to fix and added the coded from the good save file that had the quest on completed.
This is all the places where I had to add my new ID:
https://imgur.com/a/4tjEpThe only place where I didn't need to add a new <node>...</node> was the OsirisHelperVariable where I simply incremented 8958 to 8959. All the other places I had to add the nodes myself. I copied these nodes from my good save file. I made sure to keep the Handle and Object values the same as they were in the good save file, while changing the 7483 MapKey Value to 8959.
After all of this I repacked everything using the ExportTool and launched the game. I had little hope that it would work, especially since I mostly did this without any prior knowledge or testing in a text editor. But it worked on the first try! Immediately when I logged in, the quest log was updated and matched the good save's quest log. I went and spoke to Tarquin and a new dialogue line had appeared that allowed me to turn in the quest.
I can't be 100% sure that I haven't introduced some bug that will manifest itself later in the game, so do this at your own risk! For now I can say that I have played 4 hours after this and didn't notice anything wrong with the quests/game. I even went to the next act.
Again this is what worked for me. There might be easier ways to go about this, but I didn't have that much time to look at it in depth. Hopefully this will help someone who was in the same situation as me. Maybe it will also help Larian fix this bug faster.