Complex sentence: Once there, they try to locate Galbatorix and after progressing through a series of traps that take all elves save Arya prisoner, they locate Galbatorix in a huge chamber.
Split into simpler sentences: Once there, they try to locate Galbatorix. After progressing through a series of traps, at the end of which all the elves save Arya are taken prisoner, they locate Galbatorix in a huge chamber.