User with this ability either is or can transform into a demon, an evil spirit, fallen angel or Satanic divinity. Most commonly user retains their general form but with some additions; horns, claws, fangs and tail are common, as are wings, more fit (muscular and/or larger) body and color change (red, blue or black dominate) but there are several kinds of demons who are far less humanoid, varying from strange, surreal or even absolutely horrific.
For practical reasons demons can be divided into three groups defined by what they are formed from:
Physical Demons aren't necessarily solid matter, but they are material/energy beings with relatively defined/stable form and often at least somewhat human-like mind. Most users that aren't originally of demonic stock have this variation. Note that demons of this group aren't necessarily any weaker than those of the other groups, they are simply more "solid" in mind and body.
Spiritual Demons are completely spiritual beings, and thus far less bound to the laws of physics or single form. Many of them think in ways that are only tangentially relatable to humans, and their forms are both transient and often downright surreal.
Nether Demons are often both the most powerful and ancient demons, in some ways closer to sentient concepts than anything else. As such, relating to them in any way is likely impossible.
Calvin Dion Avericci (Archie's Weird Mysteries) sold his soul so he could continue to shop, thus becoming a demon bound to Mallhalla, which eventually led to him plummet down to Hell when all of his victims escaped.
Demons (Promethea) are beings from the Immateria that embody mankind's flaws and evils.
Smee (Promethea) a demon from the Immateria.
Asmoday (Promethea) a demon king that lives within the Sephirot Geburah.
Belasco (Marvel Comics) commonly referred to as the Demon Belasco, is the demonic ruler of the Limbo.
Blackheart (Marvel Comics) is a child of Mephisto, the lord of the Marvel Universe's Hell.
Demon KISS (Scooby-Doo and Kiss rock and roll mystery)
Ciel Phantomhive (Black Butler) after becoming a Demon.
Thompson, Timber and Canterbury or simply The Triplets (Black Butler) are Demon siblings.
Although he appears to be human, Czar Baldy III (Bobobo-bo Bo-bobo) is actually a demon.
Porusutoroi (Bobobo-bo Bo-bobo) obtains a demon form when he slices himself across the chest.
Nezuko Kamado (Kimetsu no Yaiba)
After his apparent 'Death' over four hundred years ago, Natsu Dragneel's (Fairy Tail) older brother Zeref brought him back to life as END (Etherious Natsu Dragneel) Zeref's strongest demon out of all his creations.
Nine Demon Gates (Fairy Tail)
Mard Geer (Fairy Tail) After transforming into his true Etherious form.
Mirajane Strauss (Fairy Tail) uses Take Over magic to take the appearance, abilities, and powers of a demonic being which is called Satan Soul.
Neo Minerva (Fairy Tail) after becoming a Demon.
Driven off the edge with Team MÄR’s interference, Kappelmeister (Marchen Awakens Romance) resorts to absorbing his own Ghost Chess henchmen into his body, obtaining a demon-like form.
The ÄRM, Allu Majuu (Marchen Awakens Romance) is a demon taking the shape of a flaming orb chained to a halberd.
Being born with a demon inside her and her sister, Mahiru Hiragi (Seraph of the End) extract the demon within her sister and took it into herself, which sped up her transformation into a complete demon.
Guardian ÄRM, Tiamat (Marchen Awakens Romance) takes on the form of the demonized mother goddess of the ocean
The Ten Commandaments (Nanatsu no Taizai) are an extremely powerful group of elite warriors and the strongest amongst the Demon Clan.
Dante and Vergil (Devil May Cry) are the twin sons of the legendary Dark Knight Sparda. As such, they possess a fraction of his demonic powers, as well as the ability to transform into full fledged demons.
Gargos (Killer Instinct 2)
Omen, the Herald of Gargos (Killer Instinct (2013))
Demise, the Demon King (The Legend of Zelda: Skyward Sword)
Ashrah (Mortal Kombat)
Sareena (Mortal Kombat)
Astaroth (Valkyrie Crusade)
Baphomet (Valkyrie Crusade) is a high ranking demon.
Demon Lord (Valkyrie Crusade)
Empusa (Valkyrie Crusade) is a dangerous dream demoness.
Flu (Valkyrie Crusade) is a demon that creates and manipulates diseases.
Mephistopheles (Valkyrie Crusade) is a demon lord with very powerful and unique alchemy and magic and posses the power to grant wishes.
Vepar (Valkyrie Crusade) is a high ranking demon.
Illidan Stormage (World of Warcraft) the Demon Hunter who absorbs the demonic powers of the Skull of Gul'dan.
Arbus (The Gray Garden) is one of the supporting characters. One of Kcalb's two demon cat underling.
Ater (The Gray Garden) is one of the supporting character. One of Kcalb's two demon cat underling.
Dialo (The Gray Garden) is one of the main characters. Chelan's best friend.
Emalf (The Gray Garden) is a demon from the Flame World. He is one of Ivlis' underlings, and one of the antagonists of the game
Ivlis (The Gray Garden) is the Devil of the Flame World and serves as the main antagonist. Father of Poemi, Adauchi, and Licorice.
Kcalb (The Gray Garden) is the Devil of the Gray World. He is one of the major characters.
Poemi (The Gray Garden) is a demon from the Flame World. She is one of Ivlis' children and one of the minor antagonists of the game.
Raspbel Preserves (The Gray Garden) is one of the supporting characters. She is Rawberry's older sibling.
Rawberry Preserves (The Gray Garden) is one of the main characters. Macarona's best friend and Raspbel's younger sister.
Reficul (The Gray Garden) appears as the "passing demon" in The Gray Garden.
Rieta (The Gray Garden) is one of the demons from the Flame World. She is one of the antagonists of the game. She is a loyal underling of Ivlis.
Yosafire (The Gray Garden) is a demon of the Gray World. She serves as the main protagonist of The Gray Garden.
Adauchi/Vendetto (Wadanohara and the Great Blue Sea) is one of the demon mercenary hired by Totsusa Kingdom to invade the Blue Sea. Poemi's older brother.
Laurentia (Wadanohara and the Great Blue Sea) is one of the mercenary demon hired by the Totsusa Kingdom to invade the Blue Sea.
Ver Million (Wadanohara and the Great Blue Sea) is a demon mercenary hired by the Totsusa Kingdom and a friend of Wadanohara.
Angron (Warhammer 40,000) was once a demigod-like being known as a Primarch, but was corrupted by Chaos and became a Daemon Prince of Khorne, the Blood God.
After being sent to Hell, Jason Voorhees (Friday the 13th: The Game) escaped, becoming a charred, trident-wielding demon in the process.