What was the name of that manhwa where a demon went in the princess body to like fix her l...

What was the name of that manhwa where a demon went in the princess body to like fix her life or something? It's a very popular one. Her hair was black and she trained the knights and stuff she has a brother that looks like her and wears glasses and the ML has dark hair too I believe

I read too many similar manhwas I can't remember which is which and I want to check if I didn't read it all the way through or if it was recently updated(￣∇￣")