"Your lives I must take in my Mistress' stead. Faith and fidelity to the end."
Sullivan was the Blade of Mistress East, Most Merciful and Stern and the Keeper of her household.
In the world of Emerald City
Dorothy convinced him she's East's acolyte as she was in possession of the Elements, golden gauntlets with magical powers. Sullivan asked her to settle the weather that was mourning after East's death.
When he realized the deception, he tried to kill Dorothy but was stopped by Lucas. As he was about to kill Lucas, Dorothy convinced him to let them both go, telling him that East killed herself. This broke his heart and he resigned to his fate, to perish with the castle tower in the tornado.