Originally Posted by Beotheric
Quote was from: https://www.pcgamer.com/baldurs-gate-3-nightsong/

But https://gamerant.com/baldurs-gate-3-bg3-should-you-save-kill-nightsong/ also says the same thing.

Basically it's more a Selune's power fades meaning Isobel can no longer maintain the shield kind of deal. Rather than a direct threat to the Inn.

Thanks but . . .

The PCGamer article, 8th August, states:
"However, slaying the daughter of Selune causes her magic to falter, dooming everyone at the Last Light Inn." The shield at the inn is down to Isobel, surely. Where do they get Nightsong being the source?

The GameRant article, 19th August, states:
"Choosing whether to press Shadowheart to kill the Nightsong will make a huge difference in how the story progresses. If Shadowheart embraces Shar completely, she will become almost evil. To fully embrace Shar is to embrace the darkness; it will now become harder to lift the curse on the shadow-cursed lands."

Why will it make it harder to lift the Shadow-Curse? The first part of that quest doesn't involve Isobel, Nightsong or Thorm directly and the final part involves killing Thorm. Killing Thorm requires either the killing or releasing of Nightsong.

I might do a bit more digging on this.