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 ash. 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, but these powers are beholden to only a few individuals and items in the galaxy. Very little is known about the Perpetuals including their origin, number, purpose or even if those suspected to be them actually being Perpetuals. 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, unless he was mutated in some way by the Arch-Traitor's powers. It could be possible as well the Golden Throne, while maintaining his life functions, is also holding his body in some sort of suspended state that prevents any changes (although this is just the speculation of a neckbeard).

Ollanius Pius

A confirmed Perpetual, ol' Ollie Persson was one of the Argonauts and fought at Austerlitz and Verdun. 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 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 millenium 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.

Given that he possibly later showed up during the Drop Site Massacre (or not, accounts are muddy here) to stab a Primarch to death we are forced to assume that he dogpaddled 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 might have been, Vulkan of the Salamanders was a confirmed Perpetual. This was put to the ultimate test by none other than the goddanm Night Haunter himself, who proceeded to perfom the sick fuckest tortures and deaths on the Lord of the Salamanders. From having him shot, cut apart, decapitated, eviscerated, impaled, dismembered, evaporated, his throat torn out with a fork, being quite literally mindfucked and spacing him naked. Vulkan did 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 build-in teleporter and send him right over to Macragge where he promptly burned up upon reentry.

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.

What raises an interesting question in Vulkan's case is regarding the source of his being. 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 trait can be passed down one's bloodline, or at least transfered by cloning.

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 would pose the question to 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.