This is episode 66 of 80 of Fate Sealed by Blood. Play it below, then continue straight into the next episode — the whole season is free and needs no account.
The closing stretch of the season, where the arc pays off.
Fu Tingyi has dragon poison, curable only by intimacy with a phoenix-marked woman—but it destroys her body. After his brother's death, his mother demands he find such a woman for an heir and insists on bringing home his brother's widow, Shen Qingqian—the world's only phoenix woman.