Complex sentence: Elsewhere, Rothbart has taken Odette to his castle lair at Swan Lake and casts a spell on her that makes her turn into a swan during the day and a human again during the night.
Split into simpler sentences: Elsewhere, Rothbart has taken Odette to his castle lair at Swan Lake. Hence, Rothbart casts a spell on her that makes her turn into a swan during the day and a human again during the night.