A villain or Anti-Hero whose primary characteristic is the fear the other side has of them.
To be the Dreaded, a character has to be far and away the most feared person in the story. These are people who make you quail not because of anything they are doing at that moment, but simply because you know who they are and what they've done before... and sometimes the latter part is optional. Maybe this character has a reputation as a Hero Killer; a person who kills the other side's strongest and noblest supporters. Maybe they use fear as a magical effect which triggers panic in others. Maybe they've done so many horrible things that nobody ever wants to speak of them again. Maybe they are simply so mind-breakingly horrible even the strongest of hearts falter. But the defining trait of the Dreaded is that they are feared. Probably has a scary name, unless they use a really scary alias instead. If their ability in combat lives up to their reputation, then the only choice one has when facing them may be to Run or Die. When/if the Dreaded character dies, his/her/its death will almost certainly be followed by much rejoicing.
Although there are exceptions, (particularly on the other side of the fence) a key characteristic of this trope tends to be invisibility. In general, characters—villains, in particular—who conform to this trope will not be seen directly the first few times they are mentioned. The show will tend to work on building up their perceived threat in the minds of the audience: a dark, shadowy something that's out there somewhere, but you don't know where, who or what they are, or even whether or not they can be killed, at least at first. It is possible for them to remain this trope after they come out of the shadows, but if they do not then beware Villain Decay.
Additionally there is a heroic equivalent called the Terror Hero, when there is a hero so renowned that the enemy would rather flee than possibly encounter them. They tend to be anti-heroes of some kind though (ranging from grade 2-4), due to audiences seldom associating dread as an emotion a pure hero inspires. Some criminals also tend to go for this in order to prevent anyone from informing on them.
Note that it isn't the audience's reaction to the Dreaded that matters, but the other characters' reactions to the Dreaded. Other characters think this person is pure Nightmare Fuel, whether they deserve this reputation or not.
See also Mook Horror Show; when The Hero is the dreaded to his enemies, this is a common way of demonstrating it. Not to be confused with characters who are merely dreaded, although both tropes may apply to the same character.
Zagreus in Big Finish Doctor Who. Even Death herself is frightened when she's suddenly humming his song, without any idea where it came from.
Zagreus sits inside your head,
Zagreus lives among the dead,
Zagreus sees you in your bed,
And eats you when you're sleeping.
Films — Animated
Rattlesnake Jake from Rango. Even the Big Bad's minions are deadly afraid when told to contact him.
Monstro from Pinocchio is known throughout the village and is famous for swallowing whole ships, even the mere mention of his name is enough to send other sea creatures fleeing.
Shere Khan from The Jungle Book. Nearly everyone is scared of him, and the rest are Too Dumb to Live. He's stripped of this status in the sequel, what with his humiliation at the hands of Mowgli, and goes full-blown Knight of Cerebus trying to get it back.
Tai Lung in Kung Fu Panda, a rogue kung fu master so scary that an entire prison was built specifically to hold him. When he escapes from said prison, everyone in the Valley is afraid of him, including his former master and The Chosen One.
Typhon likely doubles as the originalHero Killer, storming Olympus by himself and driving the gods into hiding. When he first showed up to fight Zeus, all of the other Olympians except Athena and Enyo immediately fled in terror.
Hades was this to the Greeks themselves; he's the yardstick by which terrifying creatures in myth (like Typhon) are compared to. Typhon is so bad that he was not only scarier than Hades, he frightened him.
Jörmungandr and Fenrir. When there's prophecies running around about how you're going to end the world you tend to get this.
Odin the Allfather himself. He has over 200 recorded names and titles and half of them are about what a scary guy he is.
Egyptian Mythology: Apep/ Apophis. He was so terrifying, the Egyptians built temples to prevent him from coming to them.
In Grisaia No Rakuen during the climax Yuuji invades the floating fortress of Heath Oslo. Heavily outnumbered and arriving almost unarmed, he goes out of his way to fight in as terrifying a way as possible in order to sow confusion. He baits people in and then destroys them, completely crushing the morale of his opponents.
The Irregulars from Tower of God. Normally, people who climb the Tower to get their deepest desire fulfilled have to be Chosen and are invited as Regulars. Not these guys. They came and kicked the metaphysical doors open by themselves and every time that happened, the Tower underwent drastic changes. Five Irregulars are known. One was a man-made god who defeated the entireruling structure of the Tower and then simply disappeared. Before him came a person who slew a Physical God and could create life. The third followed the former into the Tower, climbed it ten times faster than anyone elsenote it took him 50 years and built a N.G.O. Superpower that rivals the government. The fourth one was almost completely immune to magic and could copy any spell on sight. And the last one was an incredibly normal girl who stabbed her best friend in the back. People are not very fond of Irregulars. They are afraid of them.
In Our Little Adventure, Angelo invoked this feeling when he confronted Randi and Peganone during their quest to capture Umbria. He probably would have for Eva as well, but she retained her resolve due to her Paladin's fear immunity. What happened to Eva due to her not backing down and running away was accordinglyhorrifying.
In The Mansion of E, Uncle Frederick's bed-mate is called variously the Woman of Mystery or the Scary Lady.
Slade from Heart Core, The Dragon of Royce Lashiec. Not only does he love to cause even MORE pain than his master does, not only does Ame, the third strongest living demon so far, fear the mere thought of being in his precense, bu not even his own allies can be with him without fearing for their lives, knowing how he can at any point kill them just out of curiosity and boredom.
In Rusty and Co., one very well-organized gang of villains had detailed dossiers on Rusty's crew and all the people who are likely to help them. When Princess shows up and they open her file, the mugshot shows her snarling at the camera from behind her oncoming fist and the notes "DO NOT ENGAGE. EXTREMELY DANGEROUS. RUN."
Atop the Fourth Wall: Lord Vyce. Interestingly, virtually no one except Vyce himself has heard of the far more dangerous Entity he was battling.
Less eldritchy, but Todd in the Shadows visibly sinks lower every time The Nostalgia Chick tries to get him to spend time with her. Considering some of the things she's done, his fear is very justified.
SCP Foundation: SCP-173 is a living sculpture that kills its victims when they break eye contact with it. What puts it firmly in this category, however, is that 682, the regenerating lizard that just doesn't die, and hates all life on earth, only yelped in what appeared to be terror and stared at it for hours when put in the same room, merely growing new eyes when they were shot out so 173 did its thing. When you scare the crap out of this thing, you know you're feared. From Dr. Clef's 001 Proposal, this is a creature so powerful that Able refuses to go anywhere near it.
One SCP isn't an artifact but a location, an ancient prison somewhere in the desert that used to hold something horrible and only referred to as "bastard son of Apep" (see below). Used to.
Among the Slaughterhouse Nine, there is one who inspires more dread than any other, even within their ranks: Gray Boy. When Jack decided to have all the past members cloned by a factor of nine each, Gray Boy was the sole exception. Bonesaw guesses that one Gray Boy is all Jack could hope to control.
Mario Brothers has King Koopa, who is portrayed as a terrifying, nigh-unstoppable villain.