The shield at the inn is down to Isobel, surely. Where do they get Nightsong being the source?
"However, slaying the daughter of Selune causes her magic to falter, dooming everyone at the Last Light Inn."
Isobel's magic is powered by Selune, Nightsong is Selune's daughter - so killing her affects Selune and weakens her, which means her magic looses it's strength causing the shield around in the Inn to become weak enough not to keep the shadow curse out - I guess is the logic.
