Perpetual

A Perpetual is a person of the grim darkness of the 41st millenium who drew the absolutely shortest straw possible: they are immortal. And nothing of this "age without youth" or "immortality without indestructibility" bullshit: they do not age and can survive the most gruesome deaths imaginable up to and including taking an Exterminatus to the face because they can reassemble themselves from their constituent atoms. They're like Wolverine on steroids like that. Which, given the relentless sunshine of the Warhammer 40.000 universe is NOT a good thing: not even death will relieve your suffering. There are only a few things that can kill a Perpetual for good, but these powers and weapons are known to only a few individuals in the galaxy (and even then it's not known for certain how effective they are, or if they even work). Very little is known about the Perpetuals including their origin, number, purpose or even if those suspected to be them actually being Perpetuals (all that we know is that durning the Horus Heresy, there were exactly 3 Perpetuals living in Ultramar; given that Ultramar consisted of 500 planets at the time, we have to assume that they're extraordinarily rare). Of only five individuals is the status of Perpetual known or confirmed:

The Emperor

With all of his mysteries it would explain a number of things about the Emperor if he was a Perpetual: his age, knowledge in an era where everyone else on Terra was too busy LARPing a crossover of Terminator and Fist of the North Star to learn shit and his massive power (learned over time). On the other hand it would not explain why he didn't just regenerate after being bitchslapped by Horus. The best explanation for this is that that the Golden Throne, while maintaining his life functions, is also keeping him from resurrecting himself- in theory, if it was shut down, the only real effect it would have is that the Emperor would "die" for a day or two at most before fully healing from his injuries. A shame that nobody seems to realize this, as it would end most of the Imperium's problems on the spot, and would create some new ones, as quite a few people wouldn't be happy with the return of Imperial Truth, especially the Ecclesiarchy. Of course, knowing the Emperor he'd crush them like bugs if they tried to oppose him, or even if they didn't. There is no way he would agree with the current Imperium.

Ollanius Persson

A confirmed Perpetual, ol' Ollie Persson was one of the Argonauts of Greek myth and fought at Austerlitz (during Napoleon's conquests) and Verdun (infamous as one of the bloodiest battles in all of World War I). It also explains how he managed to survive boarding Horus' flagship during the Siege of Terra. It is unknown if Horus actually managed to kill him: while CHAOS MINDBULLETS certainly sound like they could do the trick it still is quite possible that he reassembled himself and walked off, not giving a fuck. A man with true balls of steel.

John Grammaticus

Agent of the Cabal John Grammaticus WHO WAS GORDON GRAMMATICUS' BROTHER WAS ONE DAY AT AN OFFICE TYPING ON A COMPUTOR is an interesting case: after he died at Anatol Hive an Eldar Autarch called Slau Dha found his body and made him into a Perpetual, signaling that it is possible for humans to become Perpetuals. That, or Slau Dha was the equivalent of Ramirez and John was Connor MacLeod from Highlander. If this were the case this would make John and Slau Dha at least eight times more awesome. Grammaticus' skills gained over the course of a millennium of life were instrumental for recruiting Alpharius to the cause of the Cabal, but he felt bad for this and stepped out of an airlock in the faint hope that it would kill him. (It did, but then he just came back to life again.)

Given that he possibly later showed up during the Drop Site Massacre or on Macragge (or not, accounts are muddy here) to stab a Primarch to death we are forced to assume that he dog-paddled through space to get all the way to Isstvan V. Which if this were the case would make him eight times more hilarious.

Vulkan

Like his dad probably was, Vulkan of the Salamanders was a confirmed Perpetual. This was put to the ultimate test by none other than the goddamn Night Haunter himself, who proceeded to perform the most depraved tortures and murders on the Lord of the Salamanders in an attempt to break his will. Whether he was shot at with bolters at point-blank range, cut apart, decapitated, eviscerated, impaled, dismembered, evaporated, getting his throat torn out with a rusty fork, being quite literally mindfucked,or being tossed naked out of an airlock, Vulkan would neither break nor die, pissing Curze right the fuck off. Eventually Curze locked him in a maze courtesy of Perturabo where he would hunt Vulkan himself. Curze's mistake was to place Dawnbringer at the center of the maze; when Vulkan got to his hammer he activated a built-in teleporter that sent him right over to Macragge where he promptly burned up in the atmosphere like a human meteor, re-killing him in the process.

Here things get fuzzy: either he got better or John Grammaticus found him and stabbed him with the Fulgurite, a spear once belonging to the Emperor himself and charged with his psychic essence. He could not regenerate from this and neither could Roboute Guilliman, Sanguinius or Lion El'Jonson remove the spear from their brother's chest. Given that he was made a casket and send back to Nocturne for burial we are forced to assume that this casket included a long tube-shaped appendage at least several feet long sticking out at an unusual angle, making this (while tragic) at least eight times more amusing (assuming that he was ever stabbed in the first place). Guilliman was hopeful that Vulkan would resurrect himself in time, and insisted that the coffin was a preservation capsule, which Guilliman dubbed "The Unbound Flame". Some Salamanders who maintained a vigil of mourning thought they could hear a heartbeat. Interesting is that The Unbound Flame is one of the nine artifacts of Vulkan searched after by the Forgefathers and is, here comes the important part, one of the four that is NOT in their possession yet. So this makes the issue even more complicated or someone stole The Unbound Flame and added it to his private collection.

What raises an interesting question in Vulkan's case is regarding the source of his powers. If it was given to him by the Emperor, it begs the question why the Emperor did not give all of his sons this ability (though it sort of answers itself with the thought of Angron becoming the Juggernaut (bitch!). If it was not the case, it would suggest that the Perpetual trait can be passed down one's bloodline, or at least transferred by cloning. Lots of Big E's abilities apparently can be passed to his 'sons' but we'll never know if he intended to pass some of them (magic powers and immortality specifically) to all of them or not.

Certainly Vulkan was probably the nicest of the primarch and as such would probably have been the best bet for an undying son. When Empy was looking over the petri dishes for which one to add the immortality to he had to choose between (in order): a two faced jerk, a preening egotistical asshole, a genocidal maniac with poor impulse control, Genghis khan, a frothing berserker, a stubborn joyless misanthrope, a psychopath, a guy with a terrible terrible secret, a nerd who'd rather be talking to a toaster, a psychopath, an aspie, a psychopath, a massively powerful psyker who can't take orders, a massive dick, a petulant child, a nice guy and two people obsessed with secrets and lies. If he did do it deliberately, he made a decent choice.

Anval Thawn

Justicar Anval Thawn of the Grey Knights is the biggest mystery of them all. All that is known is that he can walk away from the most gruesome Daemon-inflicted deaths one can imagine, much to the chagrin of his brothers and bewilderment of the Brotherhood's Librarians. It also begs the question of why him being a Perpetual was not detected during his induction into the Grey Knights, either during the tests or abnormal restoring from the involved chiurgery or the speed at which he accepted the nineteen organs. Perhaps the Perpetualness is undetectable by normal science or it manifests after a person's first death (much like in Highlander or the Planeswalker spark of Magic: The Gathering). Another option is that he might be Ollanius Pius or John Grammaticus, who were tired of dicking around for another 10.000 years and wanted a shot at becoming a Space Marine. And an even better question is how the fuck the Eldar were able to recognize him for what he was after only a short period of interaction with them.