So Nero if you didn’t know yet was confirmed to have needed a coalition of kingdoms in order to bring him down. Not sure why they decided to keep him alive and imprison him, maybe they just really really really want him to suffer, but he is alive and in one of the Grand Navy’s prisons. Now you can already tell from the first sentence that Nero is very powerful, so what kind of insane prison do you think they got him locked up in?
This is more just fun than a serious guess, but personally I think they got him in a deep underground in a small 3 meter by 3 meter box made purely out of arcanium. That box is chained up inside a massive tank filled with the same magic polluted water as the ocean with multiple white eyes swimming inside it incase Nero somehow broke the code to surviving underwater as a curse user. Surrounding this tank is a bunch of hazardous gas or material created by one of the curse users who can create hazards. Above the tank is a massive base that leads all the way down to the tank from the surface. Between the tank and the rest of the base is a large square tube with massive arcanium gates seperating multiple floors. The entire base is filled with guards who must have a lost magic at the least. There are multiple ancient magic users on the site too. The base is within layer 1 of the dark sea so Nero cannot go anywhere even if he takes the base. Ships are not allowed to stay for longer than a period of 30 minutes and are constantly guarded during that time to prevent any possible use of them as to make it extremely difficult for anyone to stow away on one while it’s at the base. A single powerful curse user is at the base at all times, they get to the base by staying below deck on a ship for the journey as well as it entering through a secret indoor dock in the base. This ship must immediately leave after dropping off or picking up a curse user. While the guards at the base are encouraged to attempt to stop Nero all together in the event of a breach they are all ordered to annihilate the base exposing everyone inside the base to the storm of the dark sea in the event that Nero cannot be incapacitated or killed by any of them. All curse users who have taken up position at this base have been trained extensively to gain the resolve to blow up the base if they have to even if it costs them their own life.
