Complex sentence: Dr. Garth agrees to exchange his life for Janet's but before he can be transformed Countess Zaleska is destroyed when Sandor shoots her through the heart with an arrow as revenge for her breaking her promise to make him immortal.
Split into simpler sentences: Dr. Garth agrees to exchange his life for Janet's. Before he can be transformed Countess Zaleska is destroyed when Sandor shoots her through the heart with an arrow as revenge for her breaking her promise to make him immortal.