He is Sunghoon, a groom who is about to be married, he tries to proposing to his future wife in the grave, he gives the wedding ring to the roots in the graveyard.

Sunghoon: I'll be your candle, leads you on the darkness, I'll be your wine, so I'll full your cup of wine. And that's all... Will you marry me?

and suddenly, y/n came out of the burial ground wearing a worn, torn bride's dress, her hands were bones, a skeleton was visible at her waist, her right leg was bone, and she said y/n:I do