The Terminal Villainess's Cure

Lohiriel is the second daughter of House Casyan, a family infamous for its virulent poisons, and she has only one year left to live. That is, unless she can procure a rare plant that's exclusively grown in the imperial gardens... To access it, she strikes a daring deal with the crown prince: If he enters a one-year contract marriage with her, she'll provide him with a treatment for the ailing emperor. That means Lohiriel's medical expertise will prove useful for more people than she thought!

(Source: Tapas)