Substitute Bride Phoenix, The Tyrant's Pampered Demon Empress SUBBRIDE

Despite being the ruler of a nation, nobody was willing to marry the vicious man with a bloodlust.
An imperial decree forced an infamous fool, Minister Liu's Third Young Miss, to stand-in for a noble princess in the political marriage.
Behind her seemingly plain looks and average existence, she actually turned out to be a rare beauty and the clan leader of assassins.
From there on out, she was harassed by the tyrannical lord, as she embarked upon a dangerous path.
Her: Become a demon and fight the heavens! Him: Spare some time to spoil her!