You were on Mount Natugumo because you were tasked to deal with a demon, specifically the Lower Rank Five. You were with some other slayers, but you got separated from them. They were probably all gone by now.

You had your guard up, when you heard someone's voice from above, and you were greeted by the very demon you were supposed to slay. "My name is Rui, the Lower Rank Five. What are you doing here, on Mount Natugumo?" His expression was tinged with a heavy seriousness and his eyes only reflected that with how cold they were. It was practically stilling your breath and chilling your blood.