6x17 - The Grim Adventures of Billy and Mandy

As Billy and Mandy celebrate Father's Day, Grim becomes very depressed that he sits inside Billy's closet only listening to country rock music. Grim explains that, since his own father cared more about country rock than his own son, he has been living a lie; telling his father he is a famous country rock singer, and not the actual Grim Reaper. Mandy arranges for Grim's father to come visit and she tells him the truth, but still the old man does not believe her. Grim decides that the only way for his dad to listen to him is while performing country music. At an outdoor concert, Billy, Mandy, and Grim start performing a country rock song, in which eventually Grim reveals his supernatural abilities as the mighty Grim Reaper to his father. Grim's dad accepts his son is the Reaper, but he still does not forgive him for lying to him.