Complex sentence: Abby Lockhart's final appearance as a series regular was in the Season 15 episode 3 "The Book of Abby" when she left with Luka and her son Joe to start a new life in Boston.
Split into simpler sentences: Abby Lockhart's final appearance as a series regular was in the Season 15 episode 3 "The Book of Abby". She then left with Luka and their son Joe to start a new life in Boston.