Complex sentence: Not much is known of the prior life of Eretria, as the only thing about her past life that is stated in "The Elfstones of Shannara" is that she is the adopted daughter of Cephelo, a Rover.
Split into simpler sentences: Not much is known of the prior life of Eretria. The only thing that is stated in "The Elfstones of Shannara" is that she is the adopted daughter of Cephelo, a Rover.