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.
Hobgoblin/Jason Philip Macendale, Jr (Marvel Comics); when empowered by Demogoblin
Magik (Marvel Comics); when in Darkchylde form
Hellspawn (Image Comics/Spawn)
Jackie Estacado (Witchblade)
Savanti Romero (Teenage Mutant Ninja Turtles)
Hinata Sugai (Ah My Buddha); via losing control of her power
Hiei (Yu Yu Hakusho)
Kurama (Yu Yu Hakusho)
Yusuke Urameshi (Yu Yu Hakusho)
Younger Tugoro (Yu Yu Hakusho)
Elder Toguro (Yu Yu Hakusho)
Hollow Ichigo (Bleach)
Meliodas (The Seven Deadly Sins)
Hendrickson (The Seven Deadly Sins)
Reactors (The Seven Deadly Sins)
Shin (Breath of Fire II)
Calypso (Twisted Metal series)
Dante (Devil May Cry series)
Vergil (Devil May Cry series)
Nero (Devil May Cry 4)
Arkham (Devil May Cry 3: Dante's Awakening)
The Order Of The Sword (Devil May Cry 4)
Rachel (Ninja Gaiden)
Genshin (Ninja Gaiden 2)
Kazuya Mishima (Tekken)
Jin Kazama (Tekken)
Jinpachi Mishima (Tekken)
Akuma/Oni (Street Fighter)
Ryu/Evil Ryu (Street Fighter)
Kairi (Street Fighter)
Sakura (Street Fighter)
Dan (Street Fighter)
Nightmare (Soul series)
Warlocks ("Warcraft series")
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.
When his body is taken over by the Hollow mask, Ichigo (Bleach) is now under the control of 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.
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
Spider-X (Marvel Comics) was mutated into a demonic spider by the dark magic of the Darkhold.
Edward Bloomberg (Marvel Comics) received his demon powers and form through the power of Neron.
Darkdevil (Marvel Comics) obtained several demon powers after being merged with the demon Zarathos.
Being reborn a specter of the Netherrealm, 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.
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 what happens when he lets the Satsui no Hadou comsume him.
Sam Winchester (Supernatural) possesses several demonic powers after drinking the blood of the demon Azazel.
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, we was able to gain complete control of the gene's influence and fully master its powers.
Jackie (Witchblade) gains his powers from being possessed by and ancient demon known only as the Darkness granting him his powers.