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Lord Montymort was also known as the "absorber of souls" and "sucker of essence" due to his ongoing scheme of capturing student wizards to add their magical essence to his own, thereby increasing his power. The device Montymort used to accomplish this was a pair of helmets connected by a tube. The absorption process consisted of Montymort placing one helmet on his own head, and the other onto the captured student. Then lumps of something (presumably magical essence) moved from the student's head to Montymort's. Apparently, the process was fatal to the student regardless of whether Montymort actually succeeded in collecting any essence.
At one point, Montymort had captured Ralph Wiggum, but was unable to collect any essence from him: "I'm not getting squat from this yo-yo." When he spied on Springwart's School of Magicry, he noticed Lisa Simpson's extraordinary abilities and decided to capture her. This posed a problem for Montymort, as Lisa was such a powerful sorceress that Montymort wouldn't be able to capture her unless she was defenseless. And in order to make that happen, he needed to separate Lisa from her wand, which meant he would need the help of a go-between.
Montymort ended up enlisting Bart Simpson to help him. Seeing Bart's and Lisa's rivalry (both as siblings and as student wizards), Montymort promised Bart that if he helped, he would get to see Lisa publicly humiliated. Bart quickly agreed, and Montymort planned to strike at the Springwart's Magic Recital.
At the recital, Lisa was about to perform the "levitating dragon trick" when Bart snuck backstage, stole her wand, and left a licorice Twizzler in its place. When the curtain went up, Lisa ordered the stagehands to release the dragon, picked up her wand, and said the levitation charm, but nothing happened. Then she noticed the licorice stick, saw Bart backstage laughing evilly, and realized what had happened.
Then Montymort, who had previously transformed into the dragon, revealed his true colors and changed back into his own form, but giant-sized. He then grabbed Lisa and put the connected helmets onto her head and his own. As the lumps started flowing from Lisa to Montymort, Lisa called to Bart for help. Now remorseful, Bart used Lisa's wand to cast a spell: "Prank be undone! Destroy the evil one!" However (possibly due to his not being specific enough), the spell zapped Bart instead of Montymort, scorching Bart and charring the wand.
Bart then charged at Montymort and stabbed him in the shin with the charred wand. Fortuitously for Bart, he hit Montymort's enchanted shin, the source of his power. Montymort shrank to normal size and died, and Slithers ate him whole in typical snake fashion, sobbing all the while.
 Behind the Laughter