Warning: SPOILERS ahead for The Walking Dead: Dead City episode 3!Jeffrey Dean Morgan has opened up about Negan's gruesome kill in The Walking Dead: Dead City episode 3. Near the end of the episode, one member of the rebel group, Luther, privately confronts Negan about being wanted by New Babylon's Marshals. This results in a skirmish where Negan cuts open Luther's forehead with a cheese grater, causing Luther to fall, impaling his head on a loose pipe. Negan is forced to finish the job by pushing Luther's head down on the pipe.

https://screenrant.com/walking-dead-dead-city-negan-cheese-grater-scene-response-explained/