One and Only[2021] -9/10 cdrama
only reason why I dont give it 9,5 is cause I am too fainthearted to ever re-watch it. Moreover it got sequel story to it I didnt see yet.

for historical cdrama its quite short about average kdrama watchtime.

Whole drama is sad love story of couple protagonists being forced to separate for multiple reasons.
It got few action scenes but it mostly focuses on court intrigues and main leads romance storyline which gets here alot attention thanks which you will grew fond of main leads and even though you know from start that their love is something impossible you just kinda hope for happy end.

Well if I had to compare it to something cdrama The King's Woman got quite alike vibe, but One and Only is at least grade better.

plot - ML is new emperors uncle protecting borders and leading strongest army of empire, but thanks it he is ostracized by ministers etc. When new emperor taked up throne ML made vow to never marry and stay without child to prove he isnt rebellious. In meanwhile he was forced to take on female disciple[FL] which is noble lady from esteemed family and is engaged to crown prince. FL stays with ML as master and disciple in his realm at borders of empire, but their age aint very apart and both grow fond of each other.

genre - romance, tragedy, historical