Fight and defeat the spider of the Lower World
Story-related; cannot be missed.
At the end of the cave area you'll fight a giant spider. There are three switches on the ground, and all you have to do to defeat the spider is press the three in a specific order. Don't forget to continually keep your staff lit with to protect you from getting killed. The switch order is: left, right, middle. You'll have to use your mask to fade out the middle one to avoid pressing it too soon.
Refer to the "Sunset" achievement solution for a video walkthrough of the whole game.