It's been awhile since I've read it so my recollection might be foggy.

The manga I'm thinking of is a cultivation xianxia setting. It's high fantasy so there's cleaving of mountains and flying on swords. I believe the premise is that there was some kind of demon lord who killed the empress in a war between demonic and orthodox factions. They were the top of their factions, so their battle basically cemented the victory/loss for each side at that time. However, the main story occurs thousands of years later, so we don't see the ramifications.

Now, the demon lord is relaxing as a small shop owner in the mortal world. It's been so long people don't even know if he's still alive, but his reputation still strikes fear in people. He's currently married to his wife, who is actually the empress he killed in the war thousands of years ago. She reincarnated and they fell in love normally. The funny thing is that neither of them know eithers' identities - that they were enemies in the past. They even hide that they are cultivators from each other, each believing the other to be a mortal without cultivation qualifications, and both are secretly plotting to get some legendary treasures like medicine to help the other become a cultivator.

Most of the story's plot stems from the reincarnated wife being the empress of a dynasty. Her kingdom constantly faces threats, sometimes too much for her to handle, and the demon lord reluctantly steps in because the scale grows large enough to affect his mortal life.

There are other female characters around him, but none have any romantic relationships with him. The MC and FL are devoted to each other. One of the female characters is a descendant of his brother, and the other is the student of an academy established by him or his friend?