Something really bad, such as horribly off-tune singing or bad poetry, being almost weaponized, or at least being traumatizing. The horribleness of it can range from giving nightmares, to blowing up cities.
Can take the form of a Brown Note, but there can be other effects than that trope. Sometimes this can be invoked with using the horrible thing as Cool and Unusual Punishment.
Can overlap with Anything But That!, Faux Horrific, Dreadful Musician and/or a humorous version of Even Evil Has Standards.
In Real Life, bad works don't really hurt people. But it's fairly common to at least describe a very bad movie as being "painful to watch", or to say that it "hurts to read" a bad book. In Japanese, one word for "ugly" is migurushii (見苦しい), literally "painful to look at".
Compare/Contrast Awesomeness Is Volatile.
Blink and you'll miss it, but in Sayonara, Zetsubou-Sensei, Itoshki-sensei's suicide kit, filled with poisons, pills and all manner of lethalities, also contains an Enya CD (though it might also just be relaxing suicide mood-music).
In Clannad Kotomi Ichinose specializes in this trope with her violin of doom.
And her weapon is the violin it only takes 0.2 before sound waves come out from the moment she takes position. The number of people she's felled is countless.
—Tomoya to students, after interrupting Kotomi's introduction.
Comics
Pictured above: in Astérix, Cacofonix's music is so bad, it's the only thing that can teach the meaning of fear to the Normans. In fact short exposure to it mentally scars them. It also summons thunderstorms and drives the wildlife away.
Film
The song "Puberty Love" in Attack of the Killer Tomatoes! is so bad, it defeats the tomatoes. Not that they were weak against music, but that this song was so awful, that even the sheet music had this effect.
A similar trick involving Slim Whitman works on the invading Martians of Mars Attacks!.
In the movie Top Secret!, after the Nazis have been psychologically and physically torturing Nick Rivers only for it not to break him...
"Do you want me to break out the LeRoy Neiman paintings?"
"No. We cannot risk violating the Geneva Convention."
Literature
Hitchhiker's Guide to the Galaxy: Vogon poetry, to the point that victims are restrained before readings, and the poetry of the Azgoths of Kria.
It's not necessarily that they're bad poets. It's just that with Feegles, if you can't get drunk on it or weaponize it, they don't have a lot of use for it.
And they've come close a few times. Monster A-Go Go is generally agreed to be one of, if not the worst movie ever featured on the show. Manos: The Hands of Fate was so god-awful that Dr. Forrester and Frank were actually apologizing to Joel for making him watch it. And Invasion of the Neptune Men was such an ordeal that Mike and the 'bots had to take turns stepping out of the theater for a few moments, and were only cheered up afterward by a surprise visit from Krankor.
Once Pearl took over, she used Hobgoblins as a punishment - mentioning "the movie" made Bobo scream, she kept it in the Ark of the Covenant, the 'Bots took turns trying to flee during the opening credits, and eventually they put up cardboard cutouts and tried to sneak away.
A story arc in Kamen Rider Double features a street musician whose singing (a rock-rap fusion he calls "spilk") is so horrible it kills birds in mid-flight and causes an earthquake. The conflict comes from his winning an American Idol-style TV show despite his competition including actually competent singers (as well as members ofAKB48 in Real Life), who hire the detective protagonist because they're sure the guy's cheating.
Sasha: Say something hideous and horrible jumps out at you, something so disgusting that it simply must die. [He pushes a button; a tiffany lamp appears.] Sasha: Ach! So... tacky... can't... look... directly at it...
Sasha has kind of a history with the things, so we might actually be looking at a reasonable justification this time...
When The Nostalgia Critic reviewed Junior, he saw the baby dream with the greenscreen face of Arnold on the baby, and it made him scream like a girl, and then vomit all over the place.
Really, this is kind of the Nostalgia Critic's whole schtick, except when he's reviewing something that's actually good or doing a top-eleven countdown.
A lot of the reviewers on his site, actually. For example Linkara in Atop the Fourth Wall, driven mad by a bad comic: "ANTI-LIFE JUSTIFIES MY HATE! Anti-life justifies my haaaaaattteee... [breaks down sobbing]"
"Break Up" by Mario feat. Gucci Mane and Sean Garrett makes his hair fall out in clumps and his nose bleed. Later he claims he's lucky it didn't give him cancer.
Diamanda Hagan follows the cast of Project Million into the "It's a Small World" ride. She doesn't take it too well.
Spoony has a few videos listed under the title of "Experience BIJ." Bij is apparently a Klingon pain ritual; the videos themselves largely consist of whatever Spoony has seen on the internet which is horribly annoying, and the goal is to see how long you can watch the videos without pausing, going to another window, or muting it. Few can make it.
The Cinema Snob experienced this in the aftermath of watching Video Violence—about three years later, as he was reviewing its sequel, he had just recovered from the debilitating effects.
During his first attempt to review Dr. Jekyll & Mr. Hyde for the NES, The Angry Video Game Nerd spoke about it being so bad that it traumatized him, and had a hard time bringing himself to playing it. Later, he would go to review the third Teenage Mutant Ninja Turtles movie, and said this about the ending:
What's the best way to end a shitty movie? With a shitty ending. One so bad that it leaves me scarred for life.
On The Critic,Roger Ebert was interviewing a potential new partner and showed him the kind of bad movies he'd have to watch. A version of Mrs. Doubtfire with Arnold Schwarzenegger in the title role fully skeletonized him.
Also Squidward's dancing is unbearable to watch that it hurts peoples eye's, and make some throw up.
A cutaway gag for Family Guy. Peter and a young girl stand in the living room, and she hands him a tape but warns him that if he watches it, he will die. Peter plays it on the VCR. The tape in question turns out to be Mannequin; in the next scene Peter dies with a contorted appearance, similar to victims of The Ring.
At one point in Futurama, Bender auditions for a part on the Show Within a Show when a role opens up. His lousy "Spanish accent" prompts this response from Calculon:
Calculon: That was so terrible, I think you gave me cancer!
