The spider caught Tove. Luckily it can't take daylight.
Use the mirrors and the hatches to gradually drive her away. Every time you point two light sources into the crystal the spider will back up some more.
It is quite easy as you get to do it. Pick up the mirrors and find where the light moves. Remember to make the double light so that you can go and open the hatches. You can do it.
It is webbed shut. Use the dagger to free it.