This is probably one of the weirdest Christmas specials ever with a twist ending! Invader Zim is a short lived Nickelodeon show about a loser alien sent to a harmless planet (Earth) so that he can be kept busy trying to conquer it while the rest of his race take their armada to destroy planets elsewhere.
The Most Horrible X-Mas Ever starts 2 million years in the future where kids are told of the awful tale of Zim and his holiday plan to destroy the world by disguising himself as Santa! But Zim's plan backfires when he realizes that the Santa suit he built reacts positively to candy canes, jiggly bells and happy, filthy child drool causing him to spread yuletide cheer.
If you are into the comic scene then you know of the twisted mind of Jhonen Vasquez and his creations Johnny the Homicidal Maniac, Squee and Fillerbunny. Invader Zim is also his creation so you can expect the same insane dialog, square-headed dirty children and moose ....so many moose.
The show also sports some pretty impressive animation. Outrageous expressions and vibrant colours dazzle the eyes and there is even some use of 3D animation in the same style as Futurama using Toon shading.
Also included in this special is the soon-to-be-classic Christmas song, Bow Down to Santa.
I find this show laugh out loud funny and this episode is no exception. It is one of Zim's most brilliant and elaborate plans and would make a great introduction for anyone who has never seen the show before. This episode can be found on Invader Zim, Volume 3: Horrible Holiday Cheer