The Undertaker/WMG

The Undertaker will play The Saint of Killers in the eventual Preacher (Comic Book) movie.
C'mon! It's only the role he's been practicing for all his life! Can we get a petition started?

Mark Calaway sold his soul to the Devil in order to survive the fire that killed his parents, and Kane is a servant of Satan sent to Earth to keep his brother in line
Which is why Vince McMahon stated in the late 90s that "Mark's beginning to think his gimmick is real" or whatever. Undertaker is to Mark Calaway as Ghost Rider is to Johnny Blaze (just that Vince wasn't aware, since he has to pose as a normal human). Kane DID die in the fire, but he was sent from Hell by Satan to make sure Undertaker did Satan's bidding. That's why Kane buried Undertaker alive for becoming "human."

Also why Undertaker started off as a jobber/lower mid-carder in WCW. He was simply wrestling as himself, and eventually realized that he could use the power of Satan to become a better wrestler, and then became The Undertaker. Since Undertaker isn't really a personification of death and more of a vicarious representation of Satan, he doesn't have his full power, which is why he's able to lose. He has some Satanic powers, but still has a human body.

The episode of Poltergeist: The Legacy that Undertaker starred in is canon in kayfabe.
Undertaker really does find souls who have escaped from hell and bring them back, at least when he's not busy wrestling. It's what he did all those times when he was supposedly out on injury, and now, it's what he does between Wrestlemanias.
 * The episode took place towards the end of 1999. When Undertaker was defeated in the episode, he regained his mortality and was able to come back to the WWF months later as his mortal self, the Bikertaker.
 * He brought Edge to Hell once, just to prove a point. Edge wasn't released until he was able to prove to the powers that be that he was actually still alive and didn't belong there.

The Undertaker and Kane are reincarnations of the original Cain and Abel.
When Abel was killed, he became the Grim Reaper and Spirit of Death. In addition to his day job, he and Cain appear in instances where sibling rivalry is prominent at an extreme level, continuing their eternal feud of jealousy and revenge. Glen Jacob Callaway was jealous that his brother Mark got more love from his mother because their mother felt jealous over the affair with Paul Bearer. The two ended up fighting and inadvertently started a fire, which consumed the house and killed their parents. It was then that the spirits of Cain and Abel entered Glen and Mark respectively; they continued their feud, this time inside a wrestling ring.
 * This theory fits nicely with the Serpent Seed theory, which states that Cain was the child of Eve and Samael, also known as the Devil, while Abel was the son of Adam and Eve. Hence, Undertaker is Abel, Kane is Cain, and Paul Bearer is Satan himself.

The Gatecrashers put The Undertaker in a coma
Come on! They were told to make an impact.
 * jossed

Glacier is the long-lost Brother of Destruction
Undertaker and Kane, the Brothers of Destruction, are often spoken of as opposites, but while Kane is identified with fire, Taker is identified with both death and lightning. One would think that Kane's opposite number would identify with ice... and what wrestler shares Taker and Kane's flair for dramatic entrances, only with an ice and snow motif? Glacier, of course! Too bad Ted Turner gave him more money than Vince McMahon would have and then threw him up against an Undertaker knock-off in Mortis; we could have seen all three Brothers of Destruction as a force to be reckoned with.
 * If Glacier is a Brother of Destruction, then what about WCW's other Mortal Kombat rip-offs Wrath and Mortis who, thanks to a bizarre crossover between Nitro and Conquest, are now part of the Mortal Kombat universe?
 * Undertaker, Kane, Glacier, Mortis, and Wrath are all originally from Outworld.

Paul Bearer killed Undertaker and Kane's parents
So, Paul Bearer claims that, as a child, Undertaker set the family home on fire and killed his parents. Taker says, Nuh-uh, my brother Kane did it, but I know it was an accident and I forgive him. So Bearer brings in Kane, who believes Taker did it on purpose and is looking for revenge, and was thought also dead in the fire. They fight a few times; then Bearer turns on Kane and sides with Taker, and all of a sudden Taker's going around talking about how yes, he did kill their parents, and he did it on purpose, and Kane sucks, and ha ha ha. And then, some months later, they're bestest buddies and tag-teaming together as if nothing happened. So what's going on? Simple: Paul Bearer was the killer of Taker and Kane's parents all along and used some kind of mind control to force Taker to admit it. This was all part of a Batman Gambit to turn Kane into his own personal engine of destruction. Undertaker and Kane discovered the truth at some point and reconciled -- and then they later put their plan for revenge into motion some years later, when Undertaker returned to his "Deadman" character with Paul once again by his side. They conspired with Paul Heyman to have the Concrete Crypt Match at Great American Bash 2004 specifically to put Paul Bearer into a position where they could kill him without his being able to escape -- hence Taker pulling the lever and burying Paul in concrete when he could have rescued him. Paul survived (as reported on WWE.com), but he learned a valuable lesson -- don't fuck with the Brothers of Destruction.
 * Then Kane was deluded and insane enough to bring Paul Bearer back in 2010...still seeing him as a father figure despite all that happened.

The Undertaker has not been portrayed by Mark Callaway for years.
Rumors about characters being replaced by different wrestlers fly about more than it actually happens. One truly bizarre conspiracy theory from around 1997, complete with its own website, purports that Mark Callaway left the then-WWF over some contrived but spiteful reason; as such, the Undertaker has since been his lookalike cousin, Ed. The website, lost even to the annals of archive.org, talked about things like how 'Taker's wife at the time didn't exist and was simply a fabrication leaked into the circles of fans on the Internet who know what Kayfabe is to create a "legit" existence for Ed that was really a worked shoot.

The Undertaker is intentionally giving up part of his primary psychological advantage.
For the majority of his WWE career, Taker has used a variation of Chopin's "Funeral March" as entrance music. But as of mid-February, he's been using the Johnny Cash classic "Ain't No Grave". Now, most any of his opponents will tell you that Taker usually wins his matches with his entrance: The darkly lit arena, the pyro, the slow, "HE COMES!"-esque walk, and the Funeral March combine to rent space in his opposition's head, putting them off their game. So why drop a song about death.....indeed, THE song about death....and replace it with a song about hope? Simple. His next opponent is Triple H, at Wrestlemania. Does he really need to psyche out an opponent at a show he's NEVER lost at, especially an opponent he's already beaten at that show? The message is obvious: He doesn't need any edge. Not this time. When you hear that trumpet (or gong, in this case) sound, he's gonna rise up out of the ground (or off the canvas), ain't no grave (or Game) gonna hold his body down.

The Undertaker is the most evil creature to ever walk this planet.
After all, the Devil still answers him with "Sir."
 * Alternate guess. The Undertaker is a Psychopomp and The Devil has to be polite if he wants to get the souls he collects. That doesn't necessary prove he's not evil but could be an out.
 * He also taught Kane everything he knows about evil. So…

TexasRed became The Undertaker after a run-in with a Texas Ranger
Mark Calloway's earliest known gimmick was that of [http://www.youtube.com/watch?v=J0lRWx3NMTg Texas Red. He was even managed by Percy Pringle] (the man who would become Paul Bearer). The song details the story about how Texas Red was "killed," and would be the appropriate origin story for The Undertaker returning to life as a dead man! Yes, the song predates the gimmick, but it's no less conceivable!

The Undertaker will return to Wrestlemania one last time...and be defeated by Santino Marella.
Ok, if he returns for a 21st run, everyone will assume that the streak will end at 25, a quarter-century. If he ends up fighting someone like Marella, everyone will assume that its a throw-away match to get him up to 25...it will be the upset of the century. My evidence for this is the efforts recently to have Santino constantly, an unexpectedly, win matches.