Aria is some kind of monster, belonging to an orgisation trying to keep in check aberrations that threaten the world, I presume. Apparently Akira is judged as such a thing. So, Aria sealed him and will be keeping an eye on him. That's how I understand it. It means she's nonetheless dedicating all of her life to just being a maid, to stay close to him. On the other hand, from Akira's point of view, Aria is a great villain who sealed his power, even though he doesn't know it. Perhaps Aria is an emotionless monster who feels neither hatred nor love, but if she actually does have emotions, including positive ones, she should be burdened by guilt.
In my opinion it's massive wasted potential the author is now ignoring the marriage promise from back then. It could be considered as Aria paying back to Akira. She took away his cheat skill, so she sacrifices herself to look after his needs as a wife. But the author aparently threw away that highly distorted relationship potential and instead is happily writing Akira to be a waste of skin.