With what he did in the second nightmare it's not too surprising that he would take over an entire city or at least try to if it gave him the power to pass the trial
Eerily similar to the group who entered the nightmare gate together.
"The Six Plagues are not the most powerful of the Defiled, and neither are they the most ancient. However, they are the most feared and loathsome. Mad Prince, Soul Stealer, Undying Slaughter, Torment, Devouring Beast, and the most harrowing of them all... the Dread Lord. Everyone on the Great River has heard their names. They are also called the Heralds of the Estuary, sometimes."
The Ravenous Fiend had been an exceedingly dire enemy, but now that he was serving Sunny, and thus enjoying the support of a benevolent, selfless, and supremely generous master… just how much more terrible would the wretched waif become?