I don't really have much of an argument for that, becauase to me peesonally it doesnt relaly matter if is was Selune specifically. All I know about DnD lore is what BG3 has presented to me and a few brief reads of the wiki.
But someone, somewhere, was pulling shenanigans. And I am extremely interested in that plot line, and wish they had kept it, especially since it gave Halsin more to do. And they clearly recorded lines for it because Halsin's VA mentioned an extremely emotional scene, and I assume this is when Halsin explains what happened with Isobel, confessing that the curse was his fault.
Based in the evidence we have. Someone pulled shenanigans at this meeting between Reithwin and the druids, and got Isobel killed by Halsin. Who they were, and why they did it, I don't know. And because that whole plotline was cut, we wont ever know most likely.
About the shenanigans in the Shadowcursed Lands look at codexes in Grymforge and the House of Hope and talk to a debtor in the House of Hope and you get a glimpse of what has happened and a reason more to despise a certain devil. I don't think, it was ever planned to have Halsin involved in the plot before the Shadowcurse. If I remember correctly, Ketheric turned to Shar after his wife died and to Myrkul for the promise to bring his daughter back. Halsin was part of the group fighting him in the Shar aera and might have accidentally killed Isobel ( in the EA version) during that timeline. I think, he said, that he is indirectly responsible for the Shadowcurse back then in EA, so I thought, Ketheric released the Shadowcurse as revenge for his daughters death. And later turned to Myrkul,when he promised to bring her back.
That works too.
I dont know if it was an in game note or a datamined thing but I feel like I remember reading about it from Halsin's perspctive. He he acted instinctively, his perspective was he was attcked, and he diddnt really know what happened, and he was really negatively afected by the wole thing. So if he killed Isobel durng he first battle of moonrise, something fishy happened either way. I think he did it with the glaive we get for saving he grove that does psycic damage. He said he never wated to see it again, and I know my Tav said something about it making her sad. Im assuming the glave was tainted magically somehow when it was used on Isobel.
But even if he was less involved, he is still involved. The plot was he killed Isobel somehow for some reason, and carries alot of guilt for it because it led to the shadow curse being relased. Thats the premise. He still wants to fix what was done. He feels bad about it. I feel like thats plenty of motivation for him to team up with us and come along, and because we helped him and became friends he continues with us. Im glad they added him as a companion, I am a Halsimp ater all, I just dont think they needed to change all that much about what they were originally gonna do with him, if thats even ths reason for the changes at all. He is a good aligned hero character after all.
