Mortal Coils (The Mortal Coils Series)

Mortal Coils (The Mortal Coils Series) by Nylund, Eric. paperback edition. ISBN: 9780765317971.

Nothing interesting ever happened to fifteen-year-old orphans Eliot and Fiona while theyve lived in the strict, oppressive household of their grandmother. A chance visit, however, reveals that there is much more to the twins. They are the offspring of a goddess and Lucifer, Prince of Darkness. Now, to settle the epic custody battle between these two families, the fallen angels create three diabolical temptations, and the gods fashion three heroic trials to test Eliot and Fiona. More than ever they need to stick together to survive and to learn how to use their budding supernatural abilities . . . for family allegiances are ever-shifting in the ancient, secret world they have entered.