Hai Shinigami to Fushi no Neko  

灰死神と不死の猫, Ash Grim Reaper and Immortal Cat

Gatto is motivated to excel as a sanctioned killer by his mentor. However, during a mission where he fails to eliminate his target, he witnesses his teammate being brutally murdered and transformed into a marionette in front of him. This harrowing experience leads him to encounter someone claiming to be a deity of death.