Complex sentence: The Abbot advises the Doctor to take the crystal to the Great One: the Doctor started this chain of events by removing the crystal in the first place, and it is up to him to put it back.
Split into simpler sentences: The Abbot advises the Doctor to take the crystal to the Great One. He suggests that, as the Doctor started this chain of events by removing the crystal in the first place, it is up to him to put it back.