The Guardians must face an old enemy of Kandrakar – Nerissa Crossnic, the corrupted ex-Guardian and the previous Keeper of the Heart. When the Oracle realized how the immense power of the Heart was corrupting Nerissa, he took it away from her and gave it to Cassidy, the former Guardian of Water. Obsessed with her lost power and blinded by jealousy, Nerissa tricked Cassidy and killed her. As punishment, Nerissa was stripped of her magic and sentenced to sleep in Mount Thanos.When Cornelia accidentally absorbed all five elements into herself, the seal on Nerissa’s tomb broke, setting her free. When Caleb is kidnapped, the Guardians must face Nerissa along with her four Knights of Vengeance. While successful in their rescue mission, they lost Luba who sacrificed herself to save them. Hoping to steal the Heart, Nerissa later attacked the girls through their dreams in order to weaken them (but failed). She was later successful in stealing the Heart back by tricking Will into giving it to her.
With the Heart now corrupted by Nerissa’s hatred, the former Guardian and her Knights of Vengeance attacked Kandrakar. Will later acquired the Star of Cassidy, a copy of the Heart from Cassidy’s ghost. With it, Will and her friends faced Nerissa in Kandrakar. In the final battle, Nerissa was killed by the Guardians. Despite their victory, Cornelia is heart-broken when Caleb left her due to their complicated relationship and him realizing her true age.