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.
