Wikisimpsons - The Simpsons Wiki
After Maude died, the Simpson family and Ned Flanders held a séance in order to contact her. While everyone joined hands around the table and concentrated to summon Maude's spirit, Bart snuck into the room dressed in a wig and women's clothing to prank Ned by pretending to be Maude. The prank turned out to be a dud, and it backfired when Maude's real ghost appeared behind Bart, startling everyone. Homer and Ned admired Maude, but Homer's admiration quickly turned lascivious. Maude's ghost then became angry and turned into a hideous ghoul, which terrified everyone except Lisa, who, oddly, wasn't afraid of the ghost's demonic appearance.
Thirty years into the future, Ned had apparently gotten remarried to Maude's ghost after Homer had accidentally killed Edna. Maude's ghost insisted that there was no God, but merely an empty void. Ned however, refused to hear the idea and dismissed it by laughing her off.
 Behind the Laughter