She didn't really "make" the Empress her mission. They can't voluntarily decide that. It's just etched in her being about what she should be doing, it's more of an existential extent. She just feels that it's what she should be doing, in her sub/conscious/-ness.
ah.. i just thought she didn't like the pervious person she needed to serve & wanted to choose a worthy person & she found that the new Empress truly needed her strength/power... that's why she choose her & made a pledge.
didn't get that it was a divine ability that ur power choose for u or some such.
That's not the case. She felt that her mission didn't lie with the Crown Princess, that's why she left. Even though it might be seen as betrayal, she did it to find out what her mission was.
on a whole this was good, but the lines r totally blurred cause the MC's relationships with other females doesn't come off platonic. but then we got the Crown Prince ＼（〇_ｏ）／it's like the author couldn't decide on whom to pair the MC with. as she also has feelings for him & is just rejecting him cause she has made the Empress her mission.
also Princess Johanna was right, they all basically fell into Nisiana's hands as she loses her power when she get far from the capital - she can't consumes anyone's powers to reach her goal - now everyone she needs is right in the capitol!
& if that wasn't enough, now that shady f*cker Marquis Venox has come to lead Johanna deeper into Nisiana's trap. i love her & i will get majorly pissed if she gets hurt cause of everyone else stupidity! why not mention the fact he shady or in cahoots with Nisiana?!? Nisiana didn't learn about the dark powers all on her own - annoying ignoring a important plot-point! #－.－)