The Grim Reaper is a non-playable character in the game. When your Sims are ready to die, he will cast a spell and that Sim will die. He appeared in version 5.3.0 .
The Grim Reaper will cast a spell to your Senior Sim, that will become a Senior sim by aging them up and then the Grim Reaper will vanish. The Grim Reaper will go out of the house. He will usually start to appear when your Senior Sim's Life Duration Circle turns red.
He visits your (Senior) Sims' house when they are in the last quarter of their life. Sometimes he can be found using your Sims objects, such as a Stereo, a Bookshelf, BBQ's, ect.