The Endbringers: Unstoppable, enormous monsters whose only goal is too kill as many people as possible. At their current rate of attrition they should destroy the world in several decades.
Leviathan is a thirty-foot tall monster with macrohydrokinesis. Let's repeat that, he is a macro water user. He has sunk entire landmasses, has destroyed entire cities, has creates tsunamis on cue. There is also a severe water crisis in the Wormverse because he is specifically aiming for and destroying sources of fresh water in the world. He is also impossibly durable, strong, and utterly relentless. Did I mention he also has super speed? When he first shows up in the story, a half a dozen capes are disabled within seconds. During the attack on Brockton Bay, the strongest capes in the world barely slow him down, and he destroys half the city, killing over forty parahumans. This is considered a victory compared to what he usually does. He may actually have the highest body count among the Endbringers as he has killed millions of people in one day. Despite all of this, he is seen as the weakest of the three.
Behemoth is a forty-five foot tall horned abomination with dynakinesis, and is the only confirmed 10 on the Brute scale. Which means he is pretty much invincible and he can turn any attack into energy that he can use against you. He can scream so loud, people are torn apart. If you get too close and he notices you, you combust. He also produces deadly radiation all of the time. This thing has killed more heroes than anything else, and turns the areas he attacks into a radiated wasteland before too long. The battle plan the heroes have when he shows up is basically to hit him from a long distance away and try their best to slow the thing down while praying for Scion to show up.
The most feared of the Endbringers is the Simurgh. A creature that appears as a freakishly tall woman with a countless number of wings fanning out from her body, some fanning out from other wings. While she is a telekinetic powerful enough to toss buildings around, that's not why she is feared. She is the world's only telepath and is a precognitive so powerful she might be approaching omniscience. Unless you are a precog as well, or a certain class of thinker, she knows every single thing you will try against her. Every attack seems to miss, and every counterattack seems to hit. She is also telepathically screaming in the heads of everyone in her range. Imagine a voice screaming so loud that it is impossible to ignore and paying attention to it only causes it to distort and rise in volume. If you remain in her presence it gets even worse, becoming a very fast pattern that drives you temporarily insane. Emotions get ramped up, your inhibitions are lowered, and you have terrible flashbacks to old traumas. A canary literally smashes its head open when it hears the Simurgh's scream. But the worse thing is her precognitive powers. To quote one victim, "Every time she shows up. Every time, people who've heard this song that's in our head? Things go wrong. They snap, they break, their lives fall apart, or they do something, and it makes something else happen, and there's a major disaster. That guy who was supposedly making a clean energy source that could power whole cities? His wife and kids got killed and he became a supervillain who made it a life goal to murder anyone who tries to better society with their powers. There were others. Over and over, every time she shows up. She never does quite as much damage as Leviathan or Behemoth, not right away, but stuff always happens later." In other words, if you are in her presence for too long, you could have been chosen by her to play a role in making the world a worse place. While the extent of the damage she has done has not been revealed, it is very telling how freaking terrified/paranoid the world is of the people who who are in her presence for too long. They have killed trusted heroes, condemned the cities the Simurgh has attacked, and forced large numbers of survivors to basically become second class citizens for the rest of their lives in fear of what she could have them do. She is arguably the Big Bad of the Wormverse, and her machinations have already nearly destroyed the world. And yes, this implies that very bad things are about to happen to Panacea and the residents of the Birdcage.
The fourth Endbringer, Khonsu. Not a physical fighter like Behemoth or Leviathan, nor a planner like the Simurgh. He teleports vast distances (as in halfway across the planet) to get from place to place, making him almost impossible to pin down. He can create fields of accelerated time that cause people to die of old age in seconds, and can be used to speed up his natural regeneration. In his first appearance, rather than the usual tactics of appear, fight until injured, and leave, Khonsu simply teleported away each time things got bad for him. He's currently been fighting for days.
And finally, the fifth/sixth Endbringer, Tohu and Bohu. Bohu is a 1300-foot-tall tower with a face, who takes over city-sized chunks of land and claims complete control. Spikes from the walls, blades in the ground, pits, anything. After she's finished with her first attack, she'll smooth the terrain out to brutally crush any injured. She fights with her sister Tohu, a woman made entirely out of tightly wound hair. Tohu has three torsos and heads, each a feminized version of a cape she picks each time she shows up. When she picks a cape's face, she gains (and possibly improves on) their powers.
The Slaughterhouse Nine: Sick and depraved serial killers with powerful superpowers who always manage to get way and rebuild their ranks. They include a invincible woman who eats you alive, a emotion controller that has made parents torture their children to death while enjoying it, a mad scientist who turned himself into a living Creepy Doll, and a creepy child surgeon who regularly gives out fates worse than death to innocent people.
Screamer: Manipulates sound over a huge range. She can imitate people's voices perfectly after a little while, to the point where she can masquerade as their own thoughts, telling them to kill themselves or similarly horrible things.
Psychosoma: Possesses people and warps them into horrible monstrosities that can easily kill you. The only way to reverse it is by killing the affected parties yourself.
Panacea's power. The public knows her as a healer, but her actual power is unlimited manipulation of biology. She can turn any microbe that lands on her skin into plagues that can kill everyone around her. She threatens to inflict uncontrollable weight gain on Skitter more than once. And what she does to Glory Girl goes well beyond that even ignoring what she did to Glory Girl's mind.
She can create new species. One of the most feared characters in the setting is a monster-spawner named Nilbog (see below). Panacea can do everything he can and more.
Clockblocker: When Gallant died, Vista saw the body. When Aegis was mashed to a literal pulp by Leviathan, to the point that he couldn't function anymore, when he died, despite his power? I got to see the remains to verify for myself. But Victoria Dallon was still alive and they didn't let us see. A select few adults and family members got to see her, they carted her off to a parahuman asylum and none of the rest of us got to say goodbye, because the end result was that fucked up.
As described in the text: A caricature. A twisted reflection of how Amy saw Victoria, the swan curve of the nape of the neck, the delicate hands, and countless other features, repeated over and over again throughout. It might even have been something objectively beautiful, had it not been warped by desperation and loneliness and panic. As overwhelming as the image and the situation had been in Amy’s mind, Victoria was now equally imposing, in a sense. No longer able to move under her own power, her flesh spilled over from the edge of the mattress and onto the floor.
As pointed out by readers, and Skitter herself, the Undersiders themselves must be terrifying to others in the city.
Bitch turns dogs into giant monsters that follow her every command. To put this in perspective, go near a very big and vicious dog. The kind of dog that inspired Cujo, that is growling with giant teeth. Now get on you knees so it is taller than you are. Picture that? Bitch can turn that into monsters as big as vans. She is known for having had her dogs tear her foster family apart, and she is very territorial. She doesn't care about you. She doesn't care if you have children with you. If you come in her territory, which is a very large part of the city, she will give her dogs the hurt command. Yes, she specifically trained her dogs how to hurt people. A dog barking in the night is a scary thing for the people in Brockton Bay.
Grue creates darkness. The darkness that is a primal fear, that everyone felt at some point in their lives. A complete and utter darkness, that also makes you deaf, and screws up your sense of touch. You are essentially transported into a complete and utter void with no sound, speech, or sight. Every now and then though, you can occasionally see a a horrifying face moving in the dark. You see, Grue can see you just fine, and he is very good at hurting people.
Tattletale claims to be a mind-reader, and it's hard to dispute. The only confirmed telepath in this setting is the Simurgh (see above), so you can see why people are scared of her. She knows everything about you. All your greatest hopes, biggest secrets, and most terrible fears. She knows what makes you tick, what makes you angry, what makes you sad. You can't hide anything from her, and she'll use it to hurt you any way she can if you cross her. It doesn't matter who you are, she will get to you. This is the girl who scares the heroes more than any other Undersider, for good reason.
Imp makes it so you aren't aware she is there, even when she is standing right in front you. How does she deal with enemies? It starts small at first. One day you start to lose things. You keep misplacing your keys, or your wallet. Than you start to lose very important things. Then the furniture starts be rearranged when you aren't watching. At this point you are probably very scared at what is happening. You check your rooms, and lock your doors. The paranoia starts to really build when you start getting getting injuries you don't remember. Paper cuts between the fingers, strange markings, bruises. Then the blood starts to show up. Gallons of blood all over your walls telling you to leave the city. Believe it or not, that is Imp being nice. If she wants to kill you, she simply slits your throat. But the scary thing is that from her enemies' perspective, their throats just appear to open up all on its own. It's probably a good thing for the mental health of the people in the city that the general public is unsure about what her power is.
Regent can take over your body. He can make you a prisoner in your own mind as he forces you to do whatever he wants. All the terrible ideas that just popped in your head? When he was with his dad, he was probably pushed to do every single one of them. Even worse, once he takes you over, he can do it again instantly over a huge radius. You can never be safe if you come anywhere near him.
Last but not least is Skitter. Oh, Skitter. Most people assume she is weak because, after all, what can bugs do? Well, one bug, not much. Two bugs, still not much. A dozen bugs, probably not much. But Skitter doesn't control a dozen, or a hundred, or a thousand bugs at a time—Skitter controls all the insects in sight ... and worse, she controls all the insects in hiding. Sneaking up on her? Impossible. Running away from her? Impossible. Fighting her? Well, let me ask you a question: how well can you fight while being blanketed with stinging insects—insects that swarm into your nose, your mouth, your ears, and worse? This isn't getting into what she can do when she doesn't hold back. She can flay you alive with her bugs, have them burrow through your eyes, and send them through small wounds to do massive internal damage. Let's make this clear. She can destroy entire ecosystems, and could probably commit genocide if she truly wanted to. As one commenter put it:
Saintsant: Fuck with Skitter, and you'll die wishing you had the breath to do so screaming.
You know the Leviathan mentioned above? Well, it was once driven off by a supervillain named Lung. So, obviously, he's one of the most powerful beings in the setting, right? Skitter has destroyed him twice, and those were relatively early in her career.
Throughout the story we have heard references to a Master class villain called Nilbog. He is spoken of as a monster and mentioned in the same breath as the Endbringers and the Slaughterhouse Nine. In the first arc-16 donation interlude we finally meet him, and he does not disappoint. What is his power? He can create mutated living creatures that have powers and seem to have feelings. They run the gambit in size, and shape but they look vaguely like people. Things like armless pregnant women that vomit up creatures and babies that have spider legs. He made things that multiply when you burn them and the government has left him alone so they don't provoke him into expanding from his little town. For the past decade he has played God with the things he creates.
What Bonesaw does to Blasto. She paralyzes him from his neck down, and completely takes control of all of his motor functions, turning him into a self-aware meat puppet. With her prehensile spine. She intends to use him to clone all of the dead members of the Nine, and maybe a few heroes.
On the subject of terrifying things that Bonesaw does, there's also what she does to Grue to cause his second trigger event. She flays the skin off his limbs, pries open his ribcage, removes some of his organs and shifts others out of place, and connects his nerves to artificial ones that cover the entire room he's in so that when the rest of the Undersiders come to rescue him, they can't reach him without adding another level of unbearable pain to the agony he is already in. As is typical of Bonesaw, she makes sure he is alive and conscious throughout.
Hookwolf's recruitment to the Nine. They convinced him to work for them while he was under the effects of a plague that screwed up his ability to remember people. If he's recovered at all, he's probably realized he's stuck working with the Slaughterhouse Nine. He's surrounded by them, probably has devices implanted in him from Bonesaw to help him survive/enforce loyalty, and he can't leave because the kill order means everyone will try to kill him.
What Bonesaw does to Cherish: lock her in a Mannequin life support chamber lashed to the bottom of Brockton Bay, paralyzed, with all the negative aspects of her power cranked up to the max.
And now, thanks to her becoming Butcher 15, she has fourteen voices in her head that very probably hate her.
What Skitter does to Valefor. Valefor's powers require that he look upon his target with his naked eye — so she fills his eyeballs with maggots to blind him. Worse, as Toast pointed out in the comments section, maggots make noises that would be conducted straight through the matter of his brain to his ears — noises that sound like chewing.
How Skitter gets revenge on Alexandria in Cell 22.4. Just imagine the feeling of thousands of bugs forcing themselves down your throat, doing their best to completely fill your lungs with their bodies and gluing themselves in place with spider silk..
Echidna. Oh Lord, Echidna. She looks vaguely like the mythical Scylla and vomits up mutated evil twins of everyone she touches. She started out as a girl named Noelle, and she's getting both bigger and more and more insane ...
Looks more like Kampe, humanoid top "half" and all.
The existence of tinkers. People whose superpower makes them genius inventors capable of building nearly anything - such as WMDs. Brockton Bay is nearly destroyed by a tinker-made weapon twice. Imagine trying to control nuclear proliferation when potentially anyone could build a nuke-equivalent out of an old vacuum.
One of The Dreaded ancient horrors in the Slaughterhouse Nine was a villain called "Gray Boy". His power? He locks his opponents in tiny loops in time — resetting every second or less to where they were before, their memories the only thing that persist as they remain trapped in a prison they cannot escape. If he is very generous, he will leave them in a loop long enough to speak, syllable after agonizing syllable, interrupted constantly by the resetting of their vocal cords and lungs; if he is not, there's nothing stopping him from lighting a match and letting them feel that first moment of horrible burning pain again and again and over and over....
Scion's rampage. To see the greatest force for good in the setting become such a nightmare is terrifying.
The Irregulars' and the freed Cauldron's prisoners' own brand of justice: at least one of them indulges in bathing in blood and they crucify a limbless parahuman for the crime of having a comfier cell than most
And for being the one whom Cauldron used to wipe memories, incidentally. Not that that excuses the actions.
As of Speck, Skitter's power has mutated. She doesn't have her pinpoint control of insects anymore, but she automatically takes control of anyone within 16 feet. Worse, said control also results in a Mind Meld, where she seemingly peer into your memories and your Shard. You also happen to be fully aware while this is happening.
Taylor controlling almost every parahuman in every Earth: "Finally, everyone was working together."
The misfired trigger events that have started happening since Scion died. Instead of the worms dancing/mating, the visions are of Scion's death and the events leading up to it, but that's not the problem. The powers aren't stable — the one guy we see turns into a self-replicating blob of ooze and tendrils — and when he's killed, the shard jumps to another person, giving a similar ability. This happens 3 times before the fourth person doesn't go berserk.
Teneral e.5: The Simurgh is back in action. And cloning something.