According to the search results, Eris leaves Rudeus at the end of volume 6 of the light novel and will not reunite with him until volume 15. She is featured in a few side chapters in between those volumes, and she is training at the Sword Sanctuary. Eris dies at the age of 74, despite her age, she was still training very energetically, one day after her morning run and after practicing with her sword, she collapses in her bed and takes her final breath and passes away. Her death signifies the end of her remarkable journey, which was characterized by personal growth, love, and an unyielding pursuit of strength. The reason behind Eriss departure from Rudeus is her desire to establish her own identity separate from her noble lineage. By relinquishing her Boreas name and embarking on her own training, she aims to define herself through her own achievements rather than relying solely on her family background. This quest for independence and self-discovery is a driving force behind her departure.