Anybody remember The Grim Adventures of Billy & Mandy? That cartoon about two kids who enslave Death. Well there is 1 episode that was only aired once on Adult Swim. It was written and produced during season 6, but it was never aired again from that one time for unknown reasons. If you're expecting this to be about an episode containing blood and murder, or anything else that extremely violent, then you're about to be disappointed. The episode contains no super-deformaties, hyper-realism or gore.
The name of the episode is Grim's Boss. The plot revolved around Grim inviting his boss to dinner to be reviewed. The episode opens at Billy's house with Grim frantically vacuuming and cleaning before Billy walks covered in mud. He is followed by Mandy. Grim becomes extremely agitated with Billy for making a mess. Mandy asks why Grim seems so worried and hyper and Grim explains that his boss is coming over for dinner to review his work and that his job lies depends on how it goes. Mandy, as usual is uncaring and apathetic towards Grim's situation and simply tells Grim to make sure to do her chores when he's finished before leaving.
The scene cuts to Grim sitting at the table with Billy's family and practically begging them not to embarrass him. The door bell rings and Grim goes to answer it. The Devil himself steps in and greets Grim cheerfully, the latter referring to him as Mr. Satan. Satan asks Grim to simply call him Lou. As expected, Billy's stupidity ruins the dinner, causing Grim to start grab him by his neck and start shaking him comedically. Despite Billy's lack of common sense, Grim ends up keeping his job nonetheless.
Lou asks to speak with Grim in private. Grim obliges and Satan reveals that he's looking for a new apprentice and asks Grim to suggest someone evil enough for the job. Grim at first suggests himself, but Satan laughs saying that Grim needs to maintain his job as Death. Reluctantly, Grim takes Lou around Endsville to find a new apprentice, introducing him to General Skarr, Eris, and Nergal Jr., each ending with disastrous, comical results.
Just as Grim and Lou are about to give up, Mandy arrives and begins bossing Grim around. Impressed Satan pulls out a personal profile of Mandy leading to a clip montage of Mandy at her worse. Having "never seen a soul more evil" than Mandy's, Satan offers her the role as his apprentice. Mandy however overthrows Satan and becomes the new ruler of Hell, enslaving both Grim and the Devil. The episode ends with a shot of Mandy ruling over Hell with an iron fist as multiple souls are seen burning in agony.