To unlock this achievement, you need to equip the full Reaper armor set along with a scythe, then kill an enemy while using them. You can obtain the full Reaper armor set by completing Rebel Missions and then trading with Pierre. A scythe can be looted from wealth chests in Francia, including the Bloodied Scythe, the Wretched Scythe, the Scythe of Tribulation, and the Scythe of Revolt, which is obtained during the Majesty in the Dark main quest.