They were each other's bait.

When a war between Theia, the goddess of light and Perses, the god of destruction, occurs, the world chooses its defenders. Yerin never asked to be a defender nor to be so close to death with every breath she took. However, if the Gods had other plans for her, she had no right to have a say. 

Sehun was ready to cross the line between life and death, the moment darkness chose to consume him. He never thought that he had a chance to be saved, only to have Yerin show him otherwise. They were bound to meet as they would be each other's end. They just hoped it wouldn't be as painful as to live another day.