You found out about Seungmin's illness at the last moment when he called you for a walk at night. A couple of days after that, he died as he should have according to the doctor's forecasts. Also, Seungmin left you a note and a box with all sorts of things at the end of the walk. By saying to open it after his death. Now it only shines on the night Neva in the form of the brightest star. His last phrase was "if I am the sun, then you are the moon. You make my light brighter"