Nikita returned to Buji Island after Awakening of Insects.
At that time, more than half a year had passed since the Black Prison incident, and most of the school's attention had shifted back to campus—such as balancing the relationship between the Alliance and the First University, coordinating interests between different factions within the school, and planning the establishment of new academies.
Naturally, the school's focus on small demons like her had weakened considerably.
Thus, she had the opportunity to return to Buji Island again.
Of course, this matter, Nikita was not voluntary.
There's a concept in economics called 'negative externality', which refers to the phenomenon where someone's actions affect others, causing them to incur additional costs without receiving corresponding benefits or compensation—in a certain sense, the Ancient One advancement of the Sea Demon King is a negative externality event for the sea demons of the four seas.
