During Act 2, a Shadow Curse inflicts the land, preventing you from progressing through certain areas in . If you try to push your way through, you will suffer massive damage, but you can circumvent the Shadow Curse by using sources of light and the right spells to move through these lands. Initially, a torch will protect you, but as the Shadow Curse gets thicker the deeper you go, it won't be enough, and it may hinder certain party members depending on their weapons and fighting style to carry one.
Ambient light can also help against the Shadow Curse early on, including high-level spells like Daylight, but again, you'll need a better solution later.
If you meet Isobel at the Last Light Inn, the Cleric, she will cast Selûne's Blessing. This blessing allows the party to travel through the less dense areas of the Shadow Curse without the use of torches or ambient light but will still do nothing to protect the party from the thicker areas.
For that, you have a couple of options, including the use of the Moonlantern or Pixie's Blessing. There is a small secret to the fight against Marcus at the Last Light Inn.
Here is how to ensure you will save Isobel in Baldur’s Gate 3. As you travel through the land in Act 2 of , a group of Fist warriors who want to delve deeper into the Curse can be discovered, setting up an ambush against a Drider and some cultists.