"You didn't think I would risk losing the battle for Gotham's soul in a fist fight with you. No, you need an ace in the hole. Mine's Harvey. I took Gotham's white knight and brought him down to our level. It wasn't hard. See madness, as you know, is like gravity. All it takes is a little push."
― The Joker, The Dark Knight
"I've demonstrated there's no difference between me and everyone else! All it takes is one bad day to reduce the sanest man alive to lunacy. That's how far the world is from where I am. Just one bad day"
User can cause madness in a person or group of people that have no mental illness, depending of the users will victims may go slightly loopy or lose total grip of reality becoming delusional, lose control over themselves, paranoid, catatonic or hostile.
The Shadowed One's Staff of Protodermis (BIONICLE, Rhotuka Launcher attachment)
SCP-012 - Bad Composition (SCP Foundation)
Homer Simpson (The Simpsons) gazed upon the painting of Dogs Playing Poker, going mad from the illogical sight.
No living being can come face to face with Pennywise's (Stephen King's It) true form of "the deadlights" without instantly going completely insane.
Thesulac Demons (Angel) drives humans insane by tormenting them with their hidden fears and insecurities.
A Lorophage demon's (Angel & Faith) feeding process typically drives surviving victims insane.
Torture from the Cruciatus Curse (Harry Potter) can cause permanent mental damage. Death Eaters tortured Neville Longbottom’s parents to the point they couldn't remember him.
Inside Doctor Psycho's (DC Universe) insanity.
Slender Man (Slender Man Mythos)
It is said by Yuuki Tenpouin (Code:Breaker) that anyone who looks into Seeker's eyes would be driven mad, due to its unbearably coldness.
Reiki, in the hands of a special power user, can cause the wielder to go insane from the overflowing powers it absorbs.
Medusa (Soul Eater) is well-known for bringing insanity through mental, physical, and magical involvement.
Ace (Justice League) accomplishes this by altering the perceptions of others.
The Joker (DC Comics) wants to show the world that they all are just one bad day away from being as insane as he is. He has driven many individuals to the depths of madness with his greatest and ongoing project being the dark knight himself.
Clownpiece (Touhou Project) explaining how she can "mess with the minds" of those who see the light of her torch.
Reisen Udongein Inaba (Touhou Project) Lunatic Moon Rabbit, Red Eyes of Madness...
... has the ability to manipulate insanity through her lunatic eyes.
Those that see Lucifer's (Lucifer) demonic face are usually driven insane.
The Flare (The Maze Runner) is the name for a deadly virus that attacks the brain. As a result, those who catch it become violent, unpredictable and insane.
SCP-012 - A Bad Composition (SCP Foundation) is a musical scroll that attracts its victims in its vicinity with hypnotic music and, when they read it, causes them to injure themselves to death.
Zofis (Zatch Bell!) used his powers to make Koko’s hidden pain and suffering overpower her kindhearted personality. As a result, Koko became sadistic and insane.
The Great Ones (Bloodborne) are a diverse race of eldritch gods that are so beyond a normal human's comprehension, seeing them or even learning of them and their ways secondhand can induce insanity.
Goro Akechi (Persona 5) can transform into Shadows berserk causing real world incidents such as mental breakdowns.
Delirium of the Endless (Vertigo/DC Comics)
The Ancients (Eternal Darkness) and their multitude of minions can all deplete humans of their sanity simply through their presence.
A Xel'lotath zombie (Eternal Darkness) draining Ellia's sanity.
According to Gaunter O'Dimm (The Witcher), anyone who has learned his true name went either insane, dead, or both.