When Princess Luna (My Little Pony: Friendship is Magic) allowed her jealousy of her sister consume her, she became the evil Nightmare Moon.
After stealing the Element of Magic and bringing it into another dimension, Sunset Shimmer (My Little Pony: Equestria Girls) was transformed into a power-mad demon hellbent on conquering Equestria and destroying Twilight Sparkle.
After become corrupted by the Equestrian magic she collected, Sci-Twi/Twilight (My Little Pony: Equestria Girls – Friendship Games) became Midnight Sparkle, a dark angel-like form obssessed with the understanding of magic.
Upon returning after falling from defeat at the Final Destination stage, Bowser's (Super Smash Bros. series) Trophy was struck by lightning, resulting in a mutation that turned Bowser into Giga Bowser.
By claiming the Black Soulstone from his brother Tyrael, Malthael (Diablo III: Reaper of Souls), names himself the "Angel of Death," where his once bright wings and form became dark and deathly in the process, if that wasn't bad enough he broke the Soulstone and infused it's demonic energies to himself.
Orochimaru (Naruto) was once one of Konohagakure's legendary Sannin. However, he became obsessed with immortality and learning with all the world's Jutsu to the point where he performed numerous modifications on his own body, resulting in his true form taking on the appearance of a giant white snake.
Obito (Naruto) having his body severely amputated and giving his eye to Kakashi, he literary became a physical wreck, after his supposed death, he became infused with Madara's Zetsus and was psychologically scarred by the denial that his love Ren died, now he is a shadow of his former-self.
Nagato (Naruto) after losing his best friend, awakening his Rinnegan Powers and using the Demon Statue, he had become a crippled dangerous maniac.
Amon (Starcraft) having a hand to play in the controlling of the Zerg, put in the overwriting purpose to the Overmind to destroy the Protoss to further his plans to reshape all of creation in his image.
Whenever Ryu (Street Fighter series) gives in to the Satsui no Hadō, he becomes Evil Ryu. In this form, Ryu's abilities are increased to unbelievable level, but at the cost of losing his humanity.
Darth Sidious (Star Wars), deformed by decades of immersion in the dark side.
Darth Sion (Star Wars) used his own pain to preserve his fractured and broken body.
Anakin (Star Wars) like most Sith indulged in the dark side of the force show it by their eye color change.
Darth Vader (Star Wars) had his legs and arms amputated shortly before having his lungs seared as he was burned alive on Mustafar. His life was saved only after intervention by his master. After his transformation into a cyborg, Vader called heavily upon the dark side to marginalize his wounds so he could again be self-sufficient.
The worst case of this transformation would be Darth Nihilus (Star Wars) who not only was completely consumed by the dark side mentally but also physically. He had become a living entity of the dark side of the Force and uncaring of all life
Lucifer (Dante's Inferno) after millennia if being in Hell, he became an abomination of creation.
After possessing the Demon Soul for a long time, Deathwing's (Warcraft) body became incapable of holding all the gathered energy and started to tear his body apart, until after his elementium plates attached to him.
Thousands of Years ago, the souls of mortals were not returning to Gaea, in an attempt to regain the souls, Gohma Vlitra (Asura's Wrath) was born, the dark spirit of Gaea.
Megidramon (Digimon) after digivolving from Wargrowlmon out of Tikato's anger became the horrible abomination of rage.
Beatrice (Dante's Inferno) shortly after accepting to be Lucifer's consort and eating a evil cursed fruit.
Illidan Stormage (Warcraft) the Demon Hunter who absorbs the demonic powers of the Skull of Gul'dan.
Arthas (Warcraft) after having claimed Frostmorne, killed Malgenis, fled in Northrend, lost what's left of his sanity, destroyed his home kingdom and claimed the frozen throne and became the Lich King.
After forsaking his duty to ferry souls to the afterlife, Davy Jones (Pirates of the Caribbean) started losing himself.
Absalom (Darksiders 2) who became corruption the day Death slayed him and his kin.
Asura (Soul Eater) was a paranoid man who gave into his fear and madness and began eating human souls and even his own weapon, thus became the first Kishin.
Tousen (Bleach) gaining the powers of a Vizard willingly being depraved...
... If it wasn't enough, he gained powers that of an arrancar and became cricket like hollow monstrosity.
Acnologia (Fairy Tail) once a human but bathed in dragons blood was able to transcend into a black terrifying dragon.
After losing his best friend Frosh, his future destroyed, he was consumed by the "shadows" and has even killed his partner Sting, and thus became the aberration of despair, Future Rogue (Fairy Tail).
Gollum (Lord of the Rings) who was once a hobbit lost and killed his friend in his obsession with the one ring and lived his life in solitude and became misshapen and it poisoned his mind for five hundred years.
Kronos/Saturn (Greco-Roman Mythology), After being imprisoned in Tartarus for a long time, deformed and burning by the very prison and it's atmosphere.
After being ripped out of his host Danny Fenton, and having ripped Vlad Plasmius' Ghost half and attempting to take control of it, Danny's ghost half became corrupted by Vlad's ghost and thus Dark Danny (Danny Phantom) was born.
For his/her life, he/she was abused, beaten and constantly neglected by his/her mother Medusa, as Crona (Soul Eater) discarded things he/she relied on, the more the Black Blood became stronger, and after killing his/her mother, it has been completed and he/she has became corrupted by madness.
After his defeat in Tempest Keep, Kael'thas Sunstrider (Warcraft) and a group of Blood Elves were employed by Kil'Jaeden and became Wretcheds.
When Jade (Jackie Chan Adventures) disobeyed Jackie about getting a tattoo, she puts the mark of the Shadowkhan on her ankle, and she was able to call the Shadowkhan, but was slowly letting the shadows take her, until she became the "Queen of the Shadowkhan"
Spider-Man (Marvel) when he is attached with the Venom Symbiotic becomes stronger but loses himself in the dark emotions.
Angel a.k.a. Sorano (Fairy Tail) has spoken of humanity being hideous and sinful, she wanted nothing more to do with the world and wanted to "Disappear into the Sky like an Angel" and as a result or her over use of her Angel Magic had become the ugliness she despised.
Zamasu (Dragon Ball Super) is a Supreme Kai apprentice who was firmly convinced that all mortals are sinful beings that had no place in his world. After taking Goku's body and fusing with his futrue self, his Fusion Zamasu form became imbalanced by the conflicting natures of the two bodies that made him, resulting in a mutation that manifested his twisted, conflicted soul on the outside.
The Archon (Darksiders 2) who opened the well, let himself open to the Corruption, killed his fellow angels refusing to relinquish the Angel Key, and thus the corruption taken a hold of his body and even replace his angel wings with defiled wings when they were chopped off.
Raiden (Mortal Combat) using his corrupted lightning to resurrect a friend of his.
With Zeus's Astral (God of War) Form with has been infected with the evil fear is a dark astral projection.
With the Devil Gene granting him incredible, Kazuya Mishima (Tekken series) is able to transform into a devil-like being with no remorse for the people around him, motivated only by power and a goal to control the world.
Nox (Wakfu) when he found the Eliacube started becoming facinated in it, later it started to slowly drive him insane from his fascination until he became a anoretic husk, he later went on a mission to collect Wakfu to go back in time to save his family, he used his time magic to preserve his already aged body to go on until it finally broke down when he failed.
The Fallen (Destiny) were once glorious beings of the light but now are nomadic pirates who ravage other worlds for their own benefit.
Gloxinia (Nanatsu no Taizai) was once the Fairy King and became one of the Ten Commandments.
Artorias (Dark Souls (Left) was once apart of the Four Knights of Gwyn, before becoming consumed by the Abyss, but for traversing through, he was gifted the epithet "Artorias The Abysswalker"
Nelo Angelo (Devil May Cry series) was once Dante's brother, Vergil, who was corrupted by the demon emperor, Mundus, into a powerful demonic warrior.
Chimera (Marchen Awakens Romance) completely descended into madness after being tortured by her husband's murderers. Accepting the power of Ghost ÄRM, Chimera rejected her humanity and became a heartless killer, as well as one of the strongest members of the Chess Pieces.