Complex sentence: Bartemius "Barty" Crouch, Junior is a Death Eater who is captured with the Lestranges, who are responsible for torturing Frank and Alice Longbottom, parents of Neville Longbottom.
Split into simpler sentences: Bartemius "Barty" Crouch, Junior, is a Death Eater. His backstory is that he was captured with the Lestranges, who tortured Frank and Alice Longbottom, parents of Neville Longbottom.