- Anathemus himself, an omnipotent, omniversal god of destruction and the one whose very existance is responsible for pretty much everything bad that happens in the Mythos in some way. The terrifying thing is that, according to SDM, Anathemus is the most powerful being in the Omniverse, bar none. And it doesn't want to be your friend.
- Essentially everything about Hellgate. He manipulated Flandre into gruesomely murdering the residents of the mansion, up to and including her own sister. He murdered Patchouli, his creator and wife, in cold blood - no, that's an understatement, he murdered her with glee. He strives for the complete obliteration of the universe and the foundation of a new one where he reigns as God. And his shade emits that damn creepy static.
- The Black Spawn will turn your best friends against you, suck you dry of life and leave your world a lifeless wasteland. The Black Spawn is the direct responsible for the existance of Hellgate (see above). The Black Spawn's core is superconcentrated despair. The Black Spawn will trap you inside itself, reliving through the most tragic moments of your life, and everyone else's life, endlessly. And did I mention that simply wounding the Black Spawn requires the intervention of a God?
- As of the Shipyard arc in Hellgate 2.3, this little shop of horrors has a new member: the Black Mass (not to be confused with the Black Spawn). Where do we start? The Black Mass pulls ships out of hyperdrive, ruins their engines and strands them in space, among an armada made of ghost ships - its previous victims. It proceeds to wait for the victims to try and escape, at which points it stops the escape attempt, overrides the ship's systems and proceeds to kill everyone inside in a series of gruesome "accidents". It can also use its armada of ghost ships as its personal army, again by hijacking their systems. It inflicts Interface Screw on both the characters (messing with their Mimics) and the players. Its appearance is a featureless dark form that blots out the sky. For crissakes, its battle "theme" is THIS! Oh, and did I mention that besides what is mentioned here, there's not a damn thing we know about it as of yet?!
- Some updates regarding the Black Mass: it is also able to switch memories around so no one person can remember a coherent version of the events involving it. It also screwed with a spaceship's logs so hard that the logs now state the ship was made of glass.
- Newer updates concerning the Black Mass: it is apparently officially known as Project Null, a virus-like entity developed by The General Gone Horribly Wrong. It had the potential to infect everything mechanical in the multiverse and posed a threat to the General himself. It was thought destroyed after a long and pain-staking war, but needless to say, like so many abominations SDM creates, it apparently refused to die for good.
- SDM's stories are rife with this, apparently. As of DEPTHS Arietta gets a battlestaff (using a crystal dropped from Akuko) which transforms even the smallest attacks into unstoppable displays of dark magic, many of which are One Hit Kills. Everything it does is nightmare fuelerrific, but most notably it once transformed Arietta's Bloody Howling into a full-fledged portal into hell, releasing a horde of wraiths that pulled an unfortunate wyvern inside while devouring it alive.
- Take away Arietta's staff and shoot it into space. I dare you.
- The Xat roleplay gives us the one whose name is now known to be Nihilius (previously nicknamed Negative), uncontested master of Nightmare Fuel. Is it any wonder he is apparently the minion of Anathemus himself?
- His "N-Reimu" incarnation can induce paralyzing existential horror in those who look at her face, is associated with swarms of disembodied hands crawling out of portals, stalks the group, and if SDM is to be believed she is on the power level of Anathemus himself.
- Nihilius is also to blame for the Dark Beast phenomenon. What is the Dark Beast phenomenon? Basically, it targets people with deep-seated emotional issues, amplifies those issues a thousandfold, and gives them the power to create small pocket dimensions (Barriers) over which they reign. Think Puella Magi Madoka Magica Witches. Since each Barrier reflects the psyche of its Beast, the results are often unsettling. Dark Beast -Ursa-'s barrier, for example, offered a disturbing glimpse into the mind of a mother Ursa Major having lost her cub. She lured hundreds of babies of every species into her Barrier - trying to find her cub, or simply hoping to fill the emptiness in her heart? Dark Beast -Arachnos-' barrier, meanwhile, was a horrifying clockwork-ruled dictatorship, a cruel perversion of both his love for machinery and his desire to protect the realm. He also turned ponies into spiders and kept them on his back. Enough said.
- And now the Canterlot Caves are infested (courtesy of Nihilius again, it seems) with walking full-body costumes of Princesses Celestia and Luna, of various degrees of shoddiness. Something about seeing an empty costume walk about and stare at you with empty eyes (naturally resisting any of your attempts to repel it in the process) is just not right.
- It's actually Princess Luna, who had not been killed by Executor, only reduced to a Nihility construct. Which does not actually make any of this any less nightmarish. Now it's the story of a kind soul stuck in a nightmarish body that terrifies all of her former friends and allies, unable to say anything for herself.
- Also in the Xat roleplay, the fate of O-Hellgate (the Hellgate of the now erased first Equestria), permanently fused to the Hellgate of the second realm for having disturbed Anathemus. A distorted version of his former self, he manifests only rarely, but seems to be in atrocious suffering when he does.
- The two are revealed to have forged an alliance and are now working together. Pretty much everything they do qualifies as Nightmare Fuel. One particularly great example of their "work" is the so-called "Hell Factory", which they tried to use to make a God Aura: a sick replica of the Rainbow Factory built primarily out of thousands of dead bodies.
- Imagine if you will, that you have bedded a beautiful girl. Now imagine it's really not a beautiful girl at all, it's a horrifying abomination from the distant stars that has brainwashed you into seeing a pretty face and soft skin where there is really a gaping maw and exposed flesh, purely to gain access to your sperm - your DNA. The next day, you - and your entire species - undergo an excruciating metamorphosis. Now turned into a repugnant fleshbeast yourself, you have only one urge remaining - to spread your seed to further planets, endlessly repeating the sad fate of your own world. Congratulations, you have made love to a Sayan.
Nightmare Fuel / Shifting Worlds Anathemus Mythos