Mephiston is the Chief Librarian of the Blood Angels Space Marine chapter, one of the most powerful psykers in the Imperium, and he bears the title "Lord of Death" for A FUCKING REASON! He's a monstrous creature in artificer armour and his sole purpose in life is to RAPE you in close combat. He often contends with the likes of Ahzek Ahriman, Njal Stormcaller, that dick, Varro Tigurius, THE SWARMLORD and
Kaldor Draigo (this guy doesn't count; he's a Mary Sue bitch) for the title of baddest psychic motherfucker in the galaxy that's still mortal. Well, technically Space Marines are functionally immortal but are killed in action long before it can be considered relevant.
Origin and History
Once upon a time, Mephiston was called Brother Calistarius, one of many librarians of the Blood Angels (and possibly the same from the Sin of Damnation book, which would also make him the same Librarian Calistarius in the Space Hulk games). You know, just your average 8-foot tall, superhuman killing machine with psychic powers and vampire-ish tendencies. Nothing that special. Then one day, he was overcome by the effects of the Red Thirst.
Once a Blood Angel is overcome with the Red Thirst, he is afflicted by the Black Rage and is inducted into the Death Company. Death Company Marines are bat-shit crazy, berserker, vampire Space Marines. Since there's no cure for the Black Rage, the only thing that can be done for the Death Company is to use them as shock troops and throw them onto the front lines of the most brutal, meat grinder, suicide assaults and let them find a glorious death on the battlefield.
During the second war for Armageddon, Brother Calistarius, as a Death Company Marine, was used to assault the Orks holding up in the Ecclesorium building of Hades Hive. As he and his fellow bat-shit crazy, berserker, vampire Space Marine brothers hacked, slashed and raped their way through the Orks' defences, they might have gotten a little bit too enthusiastic. As in, they brought the entire fucking building down on their heads.
Calistarius was the only one left alive, buried in the rubble with his body crushed and broken. Trapped, denied his glorious death, and unable to vent his rage by raping the shit out of the enemy, he was forced to sit and stew in his blind fury for seven days and seven nights. The madness of the Black Rage was tearing at his mind, screaming for blood and death, causing him to trip balls with out-of-this-world, fucked up visions that had driven all before him right off the deep end!
With nowhere to go and nothing to do, he had no choice but to stare down the visions and fight the madness. At midnight on the seventh day, he finally overcame the Black Rage and the Red Thirst and was reborn as Mephiston, LORD OF DEATH!
Having done what no Blood Angel had EVER done, he was reborn with new strength and power unlike anything he had ever felt before. Feeling the utter RAAAAAGE! and fury of Mephiston's rebirth, the thousand tons of rubble that had been crushing down on him and keeping him trapped SHIT ITSELF (how does rubble shit itself, you ask? Mephiston was angry with it, that's how!) and tried to escape, allowing Mephiston to burst free.
After this, because of his awesome power and the fact that the other Blood Angels were a bit scared of him and wouldn't refuse his application for promotion when he applied for one, he quickly rose through the ranks to the position of Chief Librarian.
The Cybele Incident
Mephiston was involved in, and helped to resolve, a conflict that nearly tore the Blood Angels Chapter apart. In the novels Blood Angels: Deus Encarmine and Blood Angels: Deus Sanguinius, a schism occurs within the chapter during a campaign against the forces of Chaos on Cybele. A young Marine, Arkio, grew wings and then got all uppity and powerful, proclaiming himself to be the reincarnation of Sanguinius, the Blood Angels' late Primarch. While many of his fellow Marines swore fealty to him, a number of others, including his older brother Rafen(his actual biological brother as well as fellow blood angel), refused to acknowledge any truth to his claim. As it turns out, they were right; the whole shebang turned out to be a typically dickish Tzeentchian plot intended to tear the Blood Angels apart and turn them to Chaos, and was orchestrated by Inquisitor Remius Stele, a traitor and agent of Chaos.
Eventually, Commander Dante, who was suspicious of the events on Cybele, sent Mephiston to assess the situation and decide if Arkio really was the reincarnation of Sanguinius. Almost immediately upon arriving, Mephiston sensed the taint of Chaos in Arkio, denounced him as a fraud, and identified Inquisitor Stele as a heretic and a traitor, because Mephiston is a fucking discerning motherfucker and was way too smart for any of those asshats. But Arkio persisted in his claims, and Mephiston, feeling unusually gracious, allowed Rafen to kill him instead of just eviscerating the little bastard himself. Still, Mephiston got to annihilate scores of other Chaos Marines and cultists while he was there, so it was a good time overall.
In Mephiston: Blood of Sanguinius it was revealed that while Mephiston had been "cured" of the Black Rage, whenever he tried to unleash his full might he would be overcome by a bout of UNSTOPABLE RAGE (seriously, he conjured miniature Warp Storms several times in this book). Furthermore, his body was slowly changing (his skin turned darker, his RAGE become harder and harder to stop, etc.)
Mephiston sought the Blade Petrific, a Xenos artifact which could amplify its user's psychic power, as an answer to his problem. The Blade was housed in the shrine world of Divinatus Prime, which had been lost to the Astronomican (Mephiston and his retinue had to jump, and traverse the Immaterium blind.) Two Chaos Socerers, vying to be the protege of a third one, decided to make a little contest: Both of them would try to manipulate Divinatus Prime's populations to wage a civil war to see if who would get the blade first.
Unfortunately, in the end one of them escaped with the blade. Just as planned. Mephiston deliberately let her to lead him to the third Chaos Socerer (whose identity he had implied to know.) Mephiston also realized that there was no salvation for him (his body had almost completely changed), but he said he "would be the Blood Angels' hope."
For All You Illiterate Folks
Mephiston, however, is not based on a vampire, regardless of the Blood Angels connection. He is based on Mephistopheles, or Mephisto, a demon of Satan, marvel comic character and one of the Seven Chief (see the connection?) Devils. Mephistopheles was the demon who turned up to take Faust's soul when he made a deal with the Devil, or Satan. When Mephistopheles first appeared to Faust, he actually advised Faust not to sell his soul. Mephistopheles didn't spend his time finding people to corrupt, but came to collect on souls from people who were close, or already, damned.
So you see, Faust is Brother Calistarius and Mephistopheles is Mephiston. Calistarius overcomes the Black Rage (his soul is already damned) and becomes Mephiston (sells his soul for great power). This theory is further reinforced by the fact that, in the Blood Angels Codex itself, it says (paraphrased) "in over coming the Black Rage, he became something much worse". As in, he sold his soul to overcome the torment of the Black Rage and in doing so became a demon, or one of the Seven Chief Devils of Satan.
This similarity is is touched upon in the Lord of Death book, in which a lost Blood Angel chaplain accuses Mephiston of losing his "soul", referencing the twin curses of the chapter as being the heart and soul of who the Blood Angels are—and the reason for their humility and nobility. Mephiston rebukes him, stating that while part of the Blood Angels' humility was acquired by their suffering, it was in fact the tenets and beliefs of Sanguinius that truly fostered it. The chaplain dismisses this and very nearly comes to realize how very much of Mephiston's "soul" is still there. In the end, Mephy proves his loyalties as Doombreed himself screams to Khorne that he cannot sway Mephiston to turn to Chaos. He is then turned into warp dust by Mephiston single-handedly.
It should also be noted that Gabriel Seth, Chapter Master of the Flesh Tearers, of all people considers Mephiston a monster and an abomination. This sentiment earned him the scorn of basically every Chapter Master of every Blood Angels' successor chapter with Malakim of the Lamenters straight up saying that Seth's boys murder more people horribly than any one of the Lamenter's Death Company. Take that as you will.
On the Tabletop
As of the 7th Edition Codex, Mephiston's suffered the nerf bat. He no longer has a Monstrous Creature-like stat line, and he's essentially now a tougher captain with Mastery Level 3 and a few extra special rules (Furious Charge, Fleet, And They Shall Know No Fear). A tear shed in pity…he also got a substantial price cut and gained Independent Character, to try and make him better point-for-point. Still, S5/T5 isn't shabby, and he has a personal power (Sanguine Sword) that can make him S10, along with the other unique Blood Angel psychic powers (like boiling a guy's blood and turning him into a living bomb). So have him cut open those vehicles and slay those Monstrous beasts if need be. And of course, he can still instakill with his force sword. With Transfixing Gaze, whenever he's in a challenge he rolls 2D6, and if the result is equal to or higher than the opponent's Leadership, they shit their pants at the sight of him and Mephiston kicks their ass with a 2+ To Hit. On the other hand, he's gone from WS7 to WS6, but that doesn't change a whole lot of rolls so it's whatever. He also has Adamantium Will, which, coupled with the Psyker rule and ML3, means he'll often nullify the shit out of lesser psykers and has good odds against more powerful ones. In summary, Mephy's gone from insanely badass to sufficiently badass; take that as you will.
He's as pissed off as an Angry Marine—on one of his quiet days.