I agree with everyone else here that this ending kinda sucked.
While I would've been pissed if she didn't choose to stay with Noche (since was the blatantly obvious choice), the way the story ended so abruptly was disappointing. There wasn't really even a fight scene. It was just "Demon Lord ex machina" and it ended. The way the main story arc (the soulmate pact) finished was also disappointing. It's good that it finally happened at least, but it wasn't much better than just having "and they lived happily ever after" with how rushed the ending became after she agreed to it. What does "making the pact" look like? It was never shown. We never even find out what happened afterward. All we know is that she's unable to leave the castle for who knows how many years since the author never bothered resolving it.
Sure the story was getting boring by this point, but trying to tie up every loose thread in a few panels is unacceptable. It needed a few more chapters to deal with the villain and show what happened after the pact was made. We never even got to see what life in the Demon World is like. Reene will be there for the next 5000 years, but we don't even know what the place looks like at all. For a character so obsessed with "freedom," she did a very good job tying herself down to a single city to the point that she had no freedom whatsoever (constantly surrounded by guards, needs to ask permission to go anywhere, isn't allowed to do anything dangerous (i.e. "fun"), etc.), then she ends up even more restricted even though it was a "good ending." It's almost like the author forgot they were writing an adventure story and rolled it into a romance/political drama story partway through.
Instead of trying to save the soulmate pact for the grand finale, the author should've resolved that much earlier on since it obviously wasn't that big of a deal, cut out all the boring political stuff, and leaned on the "adventure" aspect a lot more. Why did Reene become an adventurer if she never did any adventuring? Couldn't the author have made her travel to the Demon World by her own efforts instead of just being taken there out of nowhere? Why does she have such powerful offensive magic if she's worthless in a fight (OP damsel-in-distress MC doesn't work)? Why did she say she wanted to get stronger, then not do anything? Now that she's the second most powerful being in the world (which is dumb because she didn't earn it at all), isn't there something interesting she could be doing?