Zahariel
Zahariel was a bad-ass Librarian of the First Legion during the Great Crusade and later joined Luther in exile on Caliban due to inexplicable orders of their Primarch Lion El'Jonson, despite being one of the most idealistic Space Marines you'll ever find.
Also; if you believe half the forums on the internet a potential candidate for the secret identity of the Fallen Angel Cypher (more on that below)
No model or rules yet exist for him in Forgeworlds Horus Heresy series, and may never exist, but we can only hope.
Back Story
Before the Imperium turned up, as a child of about seven years old he and his cousin Nemiel joined the The Order (a knightly order where Lion El'Jonson was grand master) after spending a night stood to attention without moving, outside of their fortress walls... without shoes or a coat... ON A DEATH WORLD.
After a few year and a cliche training montage, he loses his mentor and hero to a Beast of the Forest, so naturally he goes on an EPIC QUEST FOR VENGEANCE!
On the road discovers Watchers in the Dark who of course tell him "BEWARE", "THE FORESTS ARE HAUNTED", "TURN BACK", "YOU HAVE A SECRET HIDDEN POWER" and all that cliched shit, he continues on his merry way and eventually slays the beast.
Oh... the beast happens to be a Calibanite Lion; the biggest, nastiest beast on the planet, and only the Lion has ever killed a Lion before. Zahariel could only accomplish this by using his INNER POWER where he managed to slow down time enough that he wouldn't get chomped to bits and was able to phase through the thing to hit its weak point.
Suffice to say this makes him a bit special, so he returns home the cliche hero and gets a sword made out of one of the Lion's teeth, and the Lion makes a few pointed remarks about him being the only other one to have a lion sword just like it.
Then they do a bit more war and eventually take over the planet Just as Planned. Also wiping out the conservative Knights of Lupus on the way, who wanted to keep the beasts around to keep their knightly way of life relevant (but apparently tried to work with Chaos after all).
Then the Emperor turns up, shit breaks loose and he becomes embroiled in a taliban plot to bomb the parade. Zahariel is innocent though and uncovers the plot before it happens, gaining himself a personal audience with the Emperor, which he subsequently forgets due to mind wipe bullshit.
Then he and his cousin get drafted into the Dark Angels of course, Zahariel being one of the very first new Calibanite space marines... perhaps the first actually guaranteed to be admitted entry due to his psychic powers.
Zahariel becomes a Librarian while Nemiel becomes a Chaplain and all was well.
...Then he got sent home with Luther and a bunch of other brothers, becoming more important, eventually sort of becoming Luther's second in command, along with the Lord Cypher (which was originally a post within the Order).
Shortly afterwards however, Nemiel gets his head punched off by his Primarch for having a minor disagreement over whether the legion can use Librarians or not. Not that it stopped Zahariel using his powers back home anyway.
So then, after much social and warp upheaval back home, Zahariel meets the Watchers in the Dark again who tell him how fucked up Caliban went, since humans killed all the beasts in the forest that were guarding (and soaking up the energies of) the tainted locations cause the planet had been hiding a dark secret since the beginning of time apparently (it is really close to the Eye of Terror), and now that shit (the Ouroboros) was waking up.
Luther (who in the mean time had been reading books from the Knights of Lupus with encouragement from the then-current Lord Cypher, believed to be one of the Knights of Lupus) discovers some Terrans doing a ritual apparently to summon a daemon. He and Zahariel go to break that shit up. Zahariel dies trying to kill it but learns its True Name in the process.
BUT Luther brings him back from the dead using "Advanced Medicine" just to ask him the creature's name so he can control it himself, using what he learned from the books of the Knights of Lupus.
Zahariel lies and says he doesn't know after realising that the Terrans were, in fact, trying to banish the creature instead of summon it.
Later, after following Cypher around for a bit to see where he's been sneaking off to, Zahariel realises that he didn't manage to kill the daemon earlier, and it comes back with a bit of a vengeance. Cypher has a spastic attack and runs away.
At some point later the Angels fall and Caliban explodes.
Cypher?
Zahariel is a strong contender for the secret identity of Cypher, though that's nowhere near resolved yet, either in the 30k fluff or 40k fluff.
Lots of circumstantial evidence points towards Zahariel becoming Cypher after the heresy,
- The prologue for Descent of Angels is Cypher self-referencing, then in the story-proper Zahariel is pretty much the protagonist for both Dark Angel Horus Heresy novels (and one audio book where the then-current Cypher shits a brick and flees from Caliban's ancient terror)
- He's always made out to be the good guy constantly contrasted against his grimdark legion.
- He's one of three people in the Horus Heresy confirmed to have both contact with AND assistance from the Watchers in the Dark (the others being Lion El'Jonson & Garviel Loken)
- While Cypher's tabletop rules do not have Psyker, Zahariel's power-set accounts for Cyphers buffs, such as the ability to slow time Matrix-style (hence Cypher's bullshit ballistic skill and Initiative) and his ability to phase through objects (so can escape from pretty much anywhere you lock him up)
- Watchers in the Dark have been demonstrated to have EXACTLY the same powers when breaking Loken out of jail too. (and then some)
- (Also remember that ALL Eldar are psychic too, the Psyker rule itself primarily applies to those that fling spells around)
- Zahariel achievements are an echo of the Lion, but made more Awesome because he's not a Primarch.
- He also owned his own Lion Sword.
- An interesting tit-bit pulled from the novels Legion and Betrayer, is that a person brought back from the dead becomes a perpetual. (John Grammaticus by The Cabal, Selene by Erebus), that could account for his longevity for ten thousand years.
- He was embroiled in an early plot to assassinate the Emperor but refused on moral grounds. Since Cypher is believed by some Fa/tg/uys to be destined to eventually kill the Emperor, it makes for a delicious irony that Zahariel could be the one to do it.
- There is a picture of Zahariel and his cousin Nemiel in the hardback edition of "Fallen Angels" by Mike Lee, Zahariel looks like 40k Cypher and has the same pattern of armour. Though this is not entirely conclusive as the Dark Angels did have a lot of guys in white robes.
Not Cypher? If then Who?
The novel Pandorax threw an interesting spanner into the mixture by introducing a NOT-Fallen Fallen. Grand Master Epimetheus was one of the Grey Knight founders along with Loken, Garro etc and was in stasis on Pandorax for ten thousand years. (Grey Knights change their names to be anti-daemon). and 40k Epimetheus is very clearly a Psyker just as Zahariel was, unlike 40k Cypher.
It's made quite clear in a conversation between Epimetheus and a Lord of Change that Epimetheus should not have existed in that timeline and that he somehow traded destinies with someone else (assumed to be the ORIGINAL Cypher). Though not actually fallen (having got out just in time), Epimetheus still considers himself "Fallen" since he really should have been, and so he avoids his 40k brethren out of shame.
Not only that, but the Lord of Change claims to be the one who corrupted half of the Dark Angels legion to become the Fallen Angels, but remarks that Epimetheus was the one who "nearly spoiled everything." (see ancient evil of Caliban) Furthermore, Epimetheus KNOWS it's True Name (possibly learned the same time Zahariel "died"?) and proceeds to banish the fucker back to oblivion.
Unfortunately however, a simple Lord of Change over a few pages of a much bigger storyline seems like an incredibly sucky way to dismiss the entity which corrupted half a Legion and tore apart a planet. So it's probably not the same thing.
On the flip side to this, if Zahariel & Knight-of-Lupus Cypher traded places, it means that Cypher was originally destined to not only escape the destruction of Caliban, but to become a Grey Knight Grand Master, meaning he has to have been "spiritually pure" (with respect to Grey Knights) and that Zahariel was originally destined to Fall with the rest of them even though he was starting to act against both Cypher & Luther. Again, this storyline has yet to be fully resolved and we have absolutely no idea what Lupus-Cypher's original intentions were/are.
No matter who ends up becoming Epimetheus, his "end" is to be captured by Abaddon during the Pandorax campaign and eventually be handed over to Fulgrim to gain his allegiance for the upcoming 13th Black Crusade - wherein Epimetheus was to be used an an "unblemished receptacle" to become the Avatar of Slaanesh.
- This therefore potentially makes him a contender for the role of the Knight in Silver trapped in Slaanesh's realm that is sometimes associated with Kaldor Draigo. In fact Epimetheus was MORE POWERFUL than Draigo and even knew what was in his future, though wouldn't tell him.
Who knows?
Knowing GW there in fact may NOT be an identity for Cypher after all, keeping his origins shrouded so they can use him as a plot device as much as they want. And it's probably for good, given how unshrounding Farsight made him like ten times less interesting. It still leaves us with the question of what the fuck happens to Zahariel, the one good Dark Angel?