The fluff says Mephiston was a Librarian who feel to the Black Rage, buried below rubble and devastation, and that was somehow able to overcome it and come back to the world of the living. That experience changed his character and he become taciturn and solitary.
This far most 40k fans know. What else is there to it? Well if you have been following the Horus Heresy you should know that "knowing names grants power". And what's behind the name Mephiston?
The name Mephiston is very likely adopted from Mephistopheles, a character from Goethe's Faustus. Faustus is a complex work, full of intricancies and hard to interpret. Some critics define Mephistopheles (aka Mephistophilis or simply Mephisto) as a devil looking to corrupt those that are damned. Moreover, he has sold his soul to the devil and is sometimes interpreted as a sort of fallen angel. Hmm. Blood Rage, geneseed flaw, Death Company. Mephiston's story of fall and rebirth. Does that ring a bell?
If that is not enough, let's dig deeper onto the etimology:
- From Hebrew: Mephitz, meaning "destroyer", and tophel, meaning "liar".
- From the Greek: Me as a negation, phos meaning light, philis meaning loving. This could be a parody of the Latin Lucifer, "light-bearer".
GW is not the only one to have been inspired by this character, and Mephisto has also appeared in many Marvel comics like X-Men, Ghost Rider, Thor or the Fantastic Four.
It's this sort of inspiration in modern culture that make th 40k universe so attractive. You can easily identify elements from well known sagas like Star Wars (too many to cite!), Dune (the God-Emperor, the Sisters of Battle) or even the Lord of The Rings (Orks anyone?), or find the inspiration from ancient cultures like Rome (Ultramarines) or the Norse legends (Space Wolves). Other are less evident and I personally enjoy finding these small nuggets of history that make the 40k world.