Complex sentence: Elli Winter was first chosen to be the Traveler from Quillan until Nevva is ready to be a Traveler, but was frightened of the stories she heard and therefore handed the task to her daughter Nevva.
Split into simpler sentences: Elli Winter was first chosen to be the Traveler from Quillan. Elli was supposed to be the Traveler until Nevva was ready, but was frightened of the stories she heard and therefore handed the task to her daughter Nevva.