Nephandi are the extremists of Entropy; they are the dumdums who put the darkness in World of Darkness. The singular form is nephandus; the OoC source of such is most likely the word nefandous, meaning "unspeakable" (as in "unspeakable evils"). A faction of EEEEEVIL mages, the Nephandi run the gamut of evil, from cackling ass-nekid doom cultists to corporate executives who attend charity banquets while ordering the covert extermination of a low-income neighborhood on the other side of the continent. Many are both. You'd be amazed how different a blood-and-viscera-drenched old man looks after a shower, while wearing a suit and tie.
WoD being as Grimdark as it is, the Nephandi are basically handed a bunch of easy victories in the fluff so they can serve their purpose of making the world terrible, despite the fact that they are kill-on-sight targets for literally every splat, with only some evil ghosts, radioactive werewolves, and aliens as allies, all of which will sell them out at a moment's notice and vice versa.
Nephandi who become evil deliberately after Awakening are called barabbi, while those born with evil Avatars are called widderslainte.
So You Wanna Be A Nephandus
Ok fine, whatever. You want to sell your soul to Entropy or whatever. That's cool. Remember that your GM will have to take control and turn your character into an NPC, or Phil Brucato will show up at their house and danse skyclade on their kitchen table until they agree to. But seriously, I wouldn't bother with Nephandi PC's. Stupid Evil is only entertaining for so long. The playable analogues of the extremist Nephandi are the moderate Euthanatoi.
Anyway, basically to become a Nephandus you have to jump into an evil vagina gate called a Caul. Then you see your own personal idea of evil. Once you betray your principles and embrace it, your soul is melted down in the oozey jacuzzi and turned into evil soup before being injected back into your body. Congratulations, you can now explore the hell-realms of deformed pseudo-reality called the Qlipphoth, in order to become even more evil and powerful. You fucking doofus.
This is also sometimes done to unwilling parties, but Nephandi don't like to do it since the mage can always just choose to die, and avoid having their Avatar permanently corrupted. And it takes a very special kind of person to kneel in mingled prayer, reverence, and masturbation before a literal manifestation of everything they consider wrong and horrible in the world in the first place.
These probably aren't still canon, because they were pretty unpopular when they were introduced in Revised, but the Nephandi have their own proprietary bag of tricks called Qlipphothic Spheres. They're basically the regular spheres, but evil, geared toward the destruction or corruption of their specific aspect of reality.
Qlipphothic Correspondence is for rending space, severing sympathetic connections, erasing data, and probably making black holes.
Qlipphothic Entropy is like regular Entropy, but eviler. It has an easy time turning the user into a lich and making things fall apart, but kinda sucks at good old-fashioned luck manipulation.
Qlipphothic Forces negates energy, creating cold or stopping kinetic energy in its tracks.
Qlipphothic Life is all about cancer powers and fleshwarping.
Qlipphothic Matter destroys regular, wholesome matter and replaces it with sanity-blasting non-euclidean crimes against physics and chemistry.
Qlipphothic Mind destroys or tarnishes memories and allows the wielder to enter a null-mind state to commune with the evil consciousness of Oblivion or something.
Qlipphothic Prime can unmake or negate other Mages achievements in the form of counterspells and turn Quintessence into unusable evil Quintessence.
Qlipphothic Spirit can kill normal spirits and call up demons, Banes, evil ghosts, and gibbering elder things from the outer darkness.
Qlipphothic Time... I'm gonna be honest, I have no idea. Probably creating time paradoxes and retconning people out of existence? Who knows. Nobody uses these fucking things.
The elusive Tenth Sphere sought by all the various factions in Mage is, according to the Nephandi, the Absolute, or Descent, which basically means the end of the universe. They get their wish in the last of the possible scenarios in the Time of Judgment supplement for the gameline. It's bleak, and not just because it's one of the more poorly-written scenarios in a book that only had one-and-a-half good ones out of the way in the first quarter of the text.
The Infernalists, who like to kill puppies for satan. Well ok, that's hardly fair. The Infernalists are actually some of the more organized and pragmatic Nephandi, bartering shrewdly with demons in ways that the Order of Hermes find suspiciously similar to their own. Since they generally meet at black-cloak-and-hood secret conferences around blood-soaked altars in isolated basements, they're not terrible at staying hidden and insinuating themselves into other organizations. It helps that they have literal armies of hapless dupes with demon pacts under their control, which they can use to distract or harry rivals at will.
The K'Lasshaa are your Lovecraftian guys. Bearded ax murderers carving the Elder Sign into their victim's foreheads, mad prophets calling down the stars themselves to wreak havoc, etc. You get the idea.
The Baphometites or "Baphies" as they are apparently sometimes called, are evil club kids and hippies who use corrupted tantra and transcendental meditation to control and violate small circles of followers. That is to say that they are date rapists, essentially.
The Heralds of Basilisk are basically a bunch of trolls trying to turn the internet into an evil dragon god through memes. This could either be rad or fucking moronic, depending on the kind of vibe your game is going for.
The Obliviates, also known as the Ex-Futurians, the Eschatonics, or the "Exies", because all Nephandic factions are named with the conventions of highschool cliques, apparently. Most Nephandi are just power-hungry doom wizards hiding their unfettered hedonism behind a thin veneer of cosmic nihilism. In other words, they talk a big game, but actually like being evil and its fruits too much to really bother with the whole "Armageddon" thing. These are the exceptions. They pursue the power to end reality, be it through wrathful gods, robot uprising, or just a superbug. They're extremely petty, however, and constantly foil each other so the world can end on their terms.
The Ironhands (no cutesy nicknames for these guys) are about the intersection of evil and industry, and are what the Traditions think the Technocracy is all about. They work people to death, build WMD's, and dump radioactive waste everywhere. Obviously opposed by heroic, idealistic technomancers, who see them as a perversion of science's potential, but also alarmingly good infiltrators. Reform-minded Technocrats had best sleep with one eye open. Obvious overlap with the Malfeans goes about as well as you'd expect.
The Mammonites, aka the Cult of the Golden Bull, are exactly what they sound like: evil money wizards. Unlike the Syndicate (who are already pretty amoral to begin with), these guys have not even the slightest desire to use their wealth for anything other than world domination. Also unlike the Syndics, they aren't necessarily technomancers, blending bleeding-edge gadgetry with old-world secret rites.
Los Sangrientos are a pretty obscure faction, focusing on Aztec blood magic twisted through the lens of esoteric Catholicism, keeping the worst traits of both. The adherents style themselves as modern-day Conquistadors. I assume. They only get like one paragraph in one book.
Similarly, the Gatekeepers are the inheritors of an ancient Sumerian astrology cult. They want to stage an alien invasion for their dark masters, which puts them directly at odds with the Void Engineers. Conversely, their similar origins mean that the two organizations often overlap dangerously. Sounds interesting, right? Too bad. M20 Book of the Fallen is already out and they're not in it.
The Kids Aren't Alright (Widderslainte)
The Widderslainte present an interesting moral dilemma to Mages. They are essentially predestined towards reality-violating magical turbo-evil, but not through any conscious choice. They are literally born evil. Is than an excuse? Does that make the inherently more or less deserving of redemption than their counterparts? These are the kinds of questions that can really make a campaign.
Originally, though, if you were born a widderlainte, sucks to suck; there's basically no cure but ripping out your soul and obliterating it to break the cycle. Kind of violates the inherent moral question there, unfortunately.
In one of the few actually pretty good retcons brought about by M20, the nature of the Widderslainte was changed. The Nephandic Avatar was changed from a sucking darkness where the Widderslainte's compassion should be, to the metaphysical equivalent of an abusive parent. Widderslainte aren't even really properly Nephandi anymore. Not necessarily, at least. But there is always a danger that the voice in the back of their head will overpower their better judgement. They don't always embrace the dark side, but it's a slippery path. Even a single Nephandus is a cosmic-level threat. Is it worth the risk to let them live?
Interesting stuff. Hell, that's halfway to being a playable character concept right there.
Names To Know
The Infernalist Archmage Jodi Blake is one of the most frequently referenced high-power NPCs in Mage. A master manipulator and genius arcanist, she has been held back from full-on Descent due to her pettiness and attachment to material rivalries.
The Unnamed (confusingly also referred to as al-Aswad, which certainly sounds like a fucking name to me) is the first of the Aswadim, who are like Oracles, but evil. He can be a neat background figure, but is infamous for the terrible Time of Judgement scenario where he shows up, sits in the Tenth Seat unopposed, basically rapes the universe to death for the rest of eternity with minimal effort, and he and all his various assorted never-before-mentioned super-Nephandi friends party on the ruined Earth forever, all while there's basically nothing the players can do but try to put together some resistance to maybe, probably restore hope in some unspecified future campaign that's probably going to be a lot more interesting..
Amanda is a semi-widderslainte who joined the Euthanatos under the tutelage of the archmage Senex (as she inherited the corrupted Avatar of one of his former students, but only after he'd performed soul-surgery on it), who managed to curb her darker impulses through training in the nature of fate and morality. Her storyline is actually some of the less-cringe inducing WoD fiction, and an arguable inspiration for the modern incarnation of the widderslainte in M20.
Like many things from Ascension, the term "Nefandi" is ported over and kept thematically similar, if not necessarily identical.
In Awakening, Nefandus is a title that essentially translates to "Nameless", meaning that a mage has been abandoned by his order or Ministry. This designation includes Banishers, reapers (including the Tremere liches), politically inconvenient apostates, people caught traffickers with the Lower Depths, and of course, the Scelesti, who are thematically the most similar to Ascension's cosmic-horror doom cultists.