Complex sentence: Kelso eventually locates the notebook, which Rapava left to his daughter just before being captured and tortured to death, which proves to be the memoirs of a young girl chosen by Stalin to be the mother of his secret heir.
Split into simpler sentences: Kelso eventually locates the notebook, which Rapava left to his daughter just before being captured and tortured to death. It proves to be the memoirs of a young girl chosen by Stalin to be the mother of his secret heir.