"Originally a Persona who ruled over death, it was turned into a big scythe made of magic crystal."
A weapon employed by members of Nymphéa known as Reapers. A Persona-user hit by the Grim Reaper will have their Persona stolen and be left in a comatose state. The stolen Personas are uploaded into the Black Book, where they're being fused into a "Macro Persona". The Reaper's Persona may also yield the Grim Reaper themselves, which increases their Magical Attack.