Complex sentence: Charlotte held Sara captive in an underground bunker, built to resemble a life-sized Doll House and was using her as an ally to do her illegal deeds, including anonymous blackmail, murders, and cover-ups.
Split into simpler sentences: Charlotte held Sara captive in an underground bunker, which she built to resemble a human-sized doll house. Charlotte used Sara as an ally to do her illegal deeds, including anonymous blackmail, murders, and cover-ups.