Sunday’s episode of The Simpsons was their 600th of the series, and was their annual “Treehouse of Horror” episode.

ANNUAL SIMPSONS TREEHOUSE OF HORROR EPISODE

This year, the showrunners brought back Frank Grimes, a character who appeared in one episode nearly 20 years ago, but had a lasting impression on the series.

Grimes starred in the show’s eighth season in an episode called “Homer’s Enemy.” He was out to get Homer, but eventually met his death by grasping high-voltage cables while mocking Homer for being bad at his job at the nuclear power plant. Since then, the character has been references multiple times, and his son Frank Grimes, Jr. has appeared on the show.

In this 600th episode, Grimes returns as a ghost of himself, and once again bring a lot of hate on Homer. The writers explain why he will team up against Homer and his family with Sideshow Bob and some other enemies of the Simpsons. “They’re forming a sinister squadron of Simpsons enemies to try to kill the Simpsons,” executive producer Al Jean told EW. “If that doesn’t make people tune in, I don’t know what else we can say.”

See the clip of Grimes’s death, which aired in May 1997, below.

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