Complex sentence: The novel begins where The Indigo Spell left off in which Alchemist Sydney Sage proclaims her love to the Moroi Adrian Ivashkov, starting their taboo relationship as Sydney's younger sister Zoe is introduced as an Alchemist tasked to watch over Sydney and to learn from her.
Split into simpler sentences: The novel begins where The Indigo Spell left off, when Alchemist Sydney Sage proclaimed her love to the Moroi Adrian Ivashkov. That starts their taboo relationship just as Sydney's younger sister Zoe is introduced as an Alchemist tasked to both watch over Sydney and learn from her.