The vampire High Lord.
Now every little shit is as strong as than The Majin, and I bet we will found at least 10 more characters like that.
What happens when we reach the Vampire King? What was the point of worrying about the Majin when every little shit that comes next is going to be just as dangerous.
The first rule to write anything should be not messing your story and warping the reality or the world you created for the sake of one character. in this case, given that you introduced the MC being so OP and you also introduced the last boss (or the second last boss) in the first chapter, author has to create a false sense of challenge introducing new characters as powerful as the last boss as if it were nothing out of the ordinary, and I hope he/she doesn't keep doing this to the point that the world is plagged with enemies that make the first heroic act of defeating the Majin seems like nothing out of the ordinary, destroying the logic of the world for the sake of a character.
Most mangas that start following this path end up being failures, just a few have succeeded.