3 ideas
Death's apprentice Mort is sent to collect Princess Keli of Sto Lat, who is due to die at an assassin's hands. Mort kills the assassin instead, and a separate bubble of reality forms around the living princess and begins shrinking toward her. Her own subjects stop noticing her, and the wizard Cutwell has to keep her remembered with posters. Mort must eventually confront Death himself over the damage done by his act of mercy.
While Death takes a holiday to try being human, fishing, getting drunk and cooking at a rib house, Mort takes over the job and turns into it. He grows colder and more certain, and starts speaking in Death's capital-letter voice. A function held long enough rewrites the identity of whoever fills it.
People on the Disc do not see Death, or the princess who ought to be dead, because perception filters out anything that contradicts what the mind expects to be true. Awareness is shaped by an agreed story of reality, not by the evidence in front of you. Something officially over becomes invisible even while it stands in the room.