Complex sentence: The quote is made just before the family first enters a mansion they intend to rent for the summer; unbeknownst to them, the mansion relies upon human sacrifice in order to periodically remodel and update itself.
Split into simpler sentences: The quote is made just before the family first enters a mansion they intend to rent for the summer. Unbeknownst to the family, the mansion requires human sacrifice in order to remodel and update itself.