my own personal halsin rewrite (it's long)
not long after getting out of the underdark and That Whole Mess, he joins the druid circle where he grew up only to find that the town of reithwin is doing some heavy expansion
their circle attempts to negotiate with the thorm family and relevant lords to no avail. his mentor is trying to teach him that they need to be patient, but he's actively watching them destroy the forest around him which is also weakening thaniel and, well, he's angry about it!
meanwhile, thaniel is trying to figure out how to cope with all this shit. there's an abandoned tiefling boy he starts to connect with and there's some spiritual merging going on there. thaniel possesses him a few times and eventually they become one indistinguishable personality. basically thaniel reflects nature, and oliver reflects civilization. halsin isn't aware of this at the time.
halsin is Big Mad at how the negotiations are going and decides that he's going to kill ketheric's daughter as a way to get his attention
instead? whoops he goes ape shit and turns to shar
shit is getting bad fast and halsin is panicking. doesn't tell anyone what he did.
over the years things get bad, but it takes time for things to really ramp up to the point of war.
harpers come in, they fight alongside the druids, the shadow curse happens, yadayada that's all the same. only difference is that ketheric kidnaps thaniel (before the shadow curse, of course), rips oliver out of him, and binds him to the shadowfell in preperation for this curse to come down. when the curse does happen, thaniel uses a small amount of his strength to save art and uses him as something of a shield/power catalyst to protect them both from the shadowfell for as long as possible.
meanwhile, halsin has 100 years to stew in the guilt he's felt and not really had anyone to go to about it. aradin's crew coming to the grove is NOT the first time he's fucked off, but it is the first time he did it so impulsively and leaving the grove in danger.
the second thaniel feels halsin come back to the forest, he uses the last of his power to launch art out of the shadowfellas well as give him enough protection to find the harpers. he was waiting for any druid he knew and trusted to come back.
getting art lucid is the same.
art tells halsin about oliver. he doesn't quite believe him because he didn't know about oliver before this. he's willing to try, but first he wanted to get thaniel out of the shadowfell since he would've been exposed to it directly ever since he pushed art out and it's made pretty explicit that as soon as the curse was lifted art would die since atp he's kind of like a very alive-looking shadow zombie.
portal fight would be roughly the same EXCEPT IT WOULD BE FARTHER AWAY FROM LAST LIGHT.
you'd still find oliver in the house, but instead of teleporting, you have to track him through the curse with random quick battles as you go.
oliver gets cornered, that convo is all the same except you have to walk oliver back to thaniel's body and they merge together in the house where they originally met. whole cutscene.
at this point isobel hears about all the shit going down and goes to thank halsin without knowing he was the one who killed her originally. they both realize it as soon as they meet and an insight check would reveal there's some sketch shit, and you can push them to talk about it. otherwise, it goes unsaid and their conversation is a bit awkward and isobel rushes to leave the room while halsin looks distraught
you can confront them both individually at this point, and with enough passed checks one or both of them will tell you what happened. isobel can be convinced to talk about it if you find her tomb, but Halsin just needs a high persuasion. Halsin expresses an insane amount of guilt while Isobel leaves it at the fact that there are more important things they have to deal with at the moment.
also in this change, you NEED aylin to be alive in order to end the shadow curse. ketheric's death stops it's spread, aylin cures the curse, and thaniel heals the land.
if you figure out that halsin killed isobel, you have the opportunity to tell dame aylin post-moonrise. from there you can either let her kill halsin or convince her to step down with a HIGH skill check. important to note that isobel won't tell her about it herself or stop her from killing him. regardless of the outcome she'll seem a bit disappointed or regretful that the other outcome didn't happen.
things stay open ended if he survives that encounter, there's no forgiveness or the refusal of it. they both move on with their lives and away from each other.
most important thing is that no matter what halsin's ass stays in reithwin
once you get to baldur's gate, isobel tells you more about her death if you learn about halsin's role in it. you have the opportunity to relay his side to her (if you learned about it). she'll be quiet and then say something about understanding how desperate he was to protect his home.
you'd get a letter from halsin at the party if he's alive, and the specific contents would depend on how his shit resolved. if things are healing it'd be just like the typical ending you get with him where refugees from the absolute's army are coming to settle and he's helping out.