Complex sentence: As explained in Natsu's backstory, Taki traveled west to pursue rumors of Toki's return, promising that she would return in two weeks; she has not returned since, prompting Natsu to travel in search of her mentor.
Split into simpler sentences: As explained in Natsu's backstory, Taki traveled west to pursue rumors of Toki's return, promising that she would return in two weeks. She has not returned since, prompting Natsu to travel in search of her mentor.