At night, one was not to set foot in the potter's field. In the afternoon? It was still best not to look back upon it. No matter what, it was a place to be avoided at any cost, for always, the reaper Qenigeous lingered. All travelers, if they could, circumvented the place. It was common knowledge that one did not travel through it unless they desired death. Or, at the very least, that one should write a will before setting out. - The Princess's Plight, Chapter 3
Added on 23 May, 2014 as Boss Repelling Reward during Tome of Mystaria Raid Event.
Reaper in Western tradition is a personification of Death - a Skeleton in hood with a Scythe.
Boss in 7th and 8th Chapters in Tome of Mystaria Raid Event.