Complex sentence: They witnessed Yvaine's fall to earth and are seeking her to consume her heart, for it is a well-known fact in Stormhold that devouring the heart of the star can bring centuries of youth to witches and cause any common human being to become immortal.
Split into simpler sentences: They witnessed Yvaine's fall to earth and are seeking her to consume her heart. For it is a well-known fact in Stormhold that devouring the heart of the star can bring centuries of youth to witches and cause any common human being to become immortal.