Considering that he's "The Deadman" and all, along with having a 20 year career in WWE, The Undertaker is all about the Nightmare Fuel, and has racked up quite a fair share of it over the years.
* The Undertaker's debut back in 1990, which signaled a major paradigm shift in the way sports- entertainment would be presented in the years to come. [[SeinfeldIsUnfunny As cheesy as the gimmick may seem nowadays]], in the candy-colored circus that was WWE during the Reagan/Bush era the 'Taker gimmick was unique - and frightening. His theme, "Dark Side", was in the beginning much less elegiac and a lot more minimalist and creepy - literally like the music played in a funeral home chapel. 'Taker also [[SilentAntagonist never spoke]] and literally tried to kill his opponents, usually by choking them. Add to that Paul Bearer's [[ButNotTooWhite pasty white skin]] and unnaturally high voice, and it was undeniable that the Undertaker's appearances truly were something to fear.
* Anything that involves his "return" after being "buried alive".
* The Undertaker's theme is considered NightmareFuel due to it beginning with the loud eerie ringing of a church bell like the beginning of AC/DC's Hells Bells but scarier, then crescendoing into the creepiest rendition of Chopin's Funeral March ever, and the rest of the music sounds like something out of a Franchise/ResidentEvil game. Scary enough for ya?
** Though whenever 'Taker is working as a face, that church bell veers away from this trope into MostWonderfulSound.
** The sound of the gongs itself, even though it is part of the [[SugarWiki/AwesomeMusic Music Of Awesome]] yet when placed in the right time, it sends chills.
* Undertaker locking Wrestling/UltimateWarrior into a [[http://www.youtube.com/watch?v=rNg95xuwuCs casket.]] Even worse, it went on and on, with various officials trying (and failing) to get Warrior out of the casket. When they finally did, it was revealed that Warrior had torn up the coffin in a desperate attempt to get out. Creepy stuff for a show aired on Saturday afternoons.
* Anytime he pulls people down below the ring.