The Grim Adventures of Billy and Mandy - Season 1 - Eps 7: Fiend Is Like Friend Without the 'R'
Billy wanders into a junkyard, where a massive hole in the earth opens up and pulls him down in. Later, Mandy and Grim get pulled in as well. They land at the center of the Earth, which is home of Nergal, a devilish creature. The lonely Nergal, who has been watching Billy and Mandy for some time, wants Grim to hand over the kids to him so they can be his friends forever. Grim happily leaves the kids behind as he returns to the surface; however, he later goes back for them because he can't handle the guilt of abandoning his friends. In the end, Grim is defeated by Nergal's electrified tentacles, but Mandy defeats Nergal merely by kicking him in the shin.
About The Grim Adventures of Billy and Mandy

Title: The Grim Adventures of Billy and Mandy
First Air Date: 2001-08-24
Last Air Date: 2007-11-09
Status: Ended
Rating: 8.045/10 (from 958 votes)
Language: EN
Seasons: 8
Total Episodes: 184
Network: Cartoon Network
Genres: Animation, Comedy, Kids
Production Companies: Cartoon Network Studios
Synopsis
The exploits of the Grim Reaper, who has been forced into being the best friend of two children. A spin-off of the show Grim & Evil.
Cast

Greg Eagles
Grim (voice)

Richard Steven Horvitz
Billy (voice)

Grey DeLisle
Mandy (voice)

Vanessa Marshall
Irwin (voice)