Users are able to acquire power from demonic forces. Often times, they're cursed with the evil from unholy items, spirits, or concepts. Sometimes their bodies are warped and twisted into a hellish parody of their previous state, becoming demons. Some are able to switch in and out of their new forms at will. Those who are too weak to control the power will lose their sanity as well as their souls.
Kazumi Mishima nee Hachijo (Tekken); as Devil Kazumi
Jinpachi Mishima (Tekken)
Calypso (Twisted Metal series)
Warlocks (Warcraft series)
Ayano Aishi (Yandere Simulator); via Demonic Ritual
Rebellion (Devil May Cry series)
Yamato (Devil May Cry series)
Force Edge (Devil May Cry series)
Evil Pieces (Highschool DxD)
Blade of the Archfiend (Ninja Gaiden)
Beast Spear (Ushio and Tora)
Freddy Krueger (A Nightmare on Elm Street) derives all of his dream-powers from a deal he made with a trio of Dream Demons before his demise.
After having the curse on his arms lifted and his Anti-Magic fused into his body by the Witch Queen, Asta (Black Clover) can transform into Black Asta by taking control of the devil within, granting him superhuman strength, speed, flight and immunity to Magic.
When his body is taken over by his Inner Hollow, Ichigo Kurosaki (Bleach) loses all control of himself to Hollow Ichigo and his power is drastically increased.
Buffy Summers (Buffy the Vampire Slayer), as well as the entirety of the Slayer line, derives her power from the essence of a shadow demon.
Edward Bloomberg (DC Comics) received his demon powers and form through the power of Neron.
Raven (DC Comics)
The Rebellion (Devil May Cry series) was able to awaken its full potential after coming into contact with the blood of its wielder, Dante...
...as well as awaken Dante's dormant potential and allow him to access his Devil Trigger.
Similarly, The Yamato (Devil May Cry series), once the weapon of Vergil, when restored by Nero's Devil Bringer...
...awakens Nero's own demonic power, including a Devil Trigger of his own.
Sparda/Force Edge (Devil May Cry series) is a demon forged sword infused with the power of the Dark Knight,Sparda. It is recognized as a weapon of god-like power. In its sleeping form it is know as Force Edge, but when infused with the power of the Perfect Amulet, is become the sword, Sparda.
The Savior (Devil May Cry 4) is a statue of Sparda's image, filled with demonic energy, allowing it to gain incredible power and create a warped reality within its own body.
Credo (Devil May Cry 4) in his demonic form after the Ascension Ceremony. He is to use his left arm to prevent taking any damage, create energy waves with his sword, fly, and summon energy spears.
Agnus (Devil May Cry 4) in his demon form after the Ascension Ceremony. He is able to summon the demons he created and used them as destructive weapons.
Pitch Black (Guardians of Childhood), after being corrupted by Dream Pirates and Fearlings, became the Nightmare King and the Bogeyman.
Created using Ajuka Astaroth's Devil Technology, the Evil Pieces (Highschool DxD) in addition to demonized the users also grants them enhanced powers and abilities, such as enhanced strength, speed or magic augmentation, depending on the pieces that is used.
The King Pieces (Highschool DxD) are nine rare Evil Pieces which can enhanced a Devil's demonic power by several fold, on a magnitude of ten to a hundred, able to potentially enhanced a Demon Lord's power to the level of an Archdemon.
The Tessaiga (InuYasha) can absorb demonic abilities to vastly increase its already destructive powers.
Spider-X (Marvel Comics) was mutated into a demonic spider by the dark magic of the Darkhold.
Darkdevil (Marvel Comics) obtained several demon powers after being merged with the demon Zarathos.
Belasco (Marvel Comics) gained immortality and immense magical ability after forging a pact with the Elder Gods, a race of extradimensional demonic entities.
Being reborn a specter of the Netherrealm, Hanzo Hasashi/Scorpion (Mortal Kombat) acquired various abilities and weapons that allow him to take on any combatant. In addition to his power, he cannot be killed and his strength increases the longer he is in the Netherrealm.
Blade of the Archfiend (Ninja Gaiden) which is forged from a meteorite is further empowered by Vazdah's demonic magic.
Rachel (Ninja Gaiden)
The Ultra-Robots (Samurai Jack) were the deadliest assassins created to kill Jack. They were given the energy of the immortal demon, Aku, in order to so.
Nightmare (Soul series) wields the terrible demonic sword, Soul Edge. With each person that is slain by the blade, or every one of it's shards retrieved, the blade's power, as well as Nightmare's, increases.
Evil Ryu (Street Fighter series) is the result of what happens when Ryu lets the Satsui no Hadou consume him.
After being feed the blood of Azazel, a Prince of Hell, as a baby, Sam Winchester (Supernatural) possessed various psych related powers - premonitions, telekinesis, and demon smiting.
Kazuya Mishima (Tekken series) possesses the Devil Gene, a genetic abnormality of the Mishima bloodline. With the power of the gene, he is able to acquire the physique and abilities of a devil-like being, and gain incredible strength.
Jin Kazama (Tekken series) inherited the the Devil Gene from his father, Kazuya Mishima. However, he was able to gain complete control of the gene's influence and fully master its powers.
Jackie Estacado (Top Cow) gains his powers from being possessed by an ancient demonic force known only as the Darkness.
Ushio Aotsuki (Ushio and Tora) becomes empowered with Yokai energy whenever he wields the Beast Spear.
In season 1, Dark (Hardcore Leveling Warrior) is capable of using demonic spells after entering Great Demon x Warlock Mode.
In Season 2, Dark (Hardcore Leveling Warrior) is able to use demonic skills in his Demonic Form