We once again find ourselves amongst the servants of Darkness, however this time it is not the Dark Angels that we shall talk about, but the Blood Angels!
Needless to say , we all have read the story about how brother Calistarius was crushed beneath the rubble of an ecclisarium building on the hive world of Hades during the war for Armagedon. It is then said that for 6 days he fought off the black rage while fighting for his life under the debris of the rubble, and on the 7th day he burst our from that rubble, fully reborn.
Clearly the word Mephiston stems from a variation of Mephistopheles, who happens to be none other then the Devil himself! Hell, hes even seen as a demon in the Germanic faust stories.
Let us keep in mind that Mephistopheles comes from a greek derived name of Mephstophilis who would be broken up into 3 words: Me (me) phosto (Light) philis (friend), hence i am a friend of the light, which seems to innuate the fact that a creature of darkness and power claims to be a friend of the light, hence a force for good. It is no surprise that the codex: BA on page 47 states that according to the fluff, there is a possibility that the price paid by brother Calisatius was possibly demonic possesion or something else that replaced the black rage. Hence Mephiston could be a puppet for the powers of darkness pretending to be fighting for the imperium.
In hebrew mephiz means "Liar" while tophel means "destroyer". This once again seems to portray the possibility of Mephiston beeing a deceiver to his chapter while at the same time beeing a force of destruction that is to be reckoned with.
Now what is the most interesting is the fact that the Faust legend of Germanic folklore tells us of a story how a man (Faust) wagers his soul against the devil for power. The link is made clear when we find out that faust would have died anyway had he not made the deal, and that after the deal hes quite not the same. Which according to Mephiston entry also applies, as it clearly states that ever since his ascension to "lord of death" he has been polar opposites with his former self.
In the Faust legend, Mephistopheles warns the Summoner (Faust) of the mistake hes making, as he regrets serving Satan, and claims that heaven is better. This could be a hint to us that Mephiston could not possibly be possesed by a demon of chaos but a more human aspect, perhaps its his own darker ego, or that of his gene-father Sanguinious.
It should also be noted that there is a time limit before Mephistopheles will come to claim the soul he so un-enthusiasticaly bought. Hence it stands to reason that Mephiston too has a time limit if there is something dark festering within him.
It should also be noted that variations of the name Mephistopheles are used Games (Diablo2 and Dungeons and Dragons) Television (Digimon) and Music (Moonspell as well as Epica).
Verdict: Leaning towards a Fact
Reason: GW has done a fair job with attempting to severe ties with the original lore accounting the stories and name of Mephistopheles, how ever if you have the resources and the ability to think logicaly, youlre be able to connect the dots, that Mephistopheles is Mephiston, where as Calistarius is the germanic character Faust.