Red Gnosis (The Legend of Heroes: Zero no Kiseki/Ao no Kiseki)
The Nightosphere amulet (Adventure Time) transforms the wearer into a powerful demon.
The God Hand (Berserk) can transform humans who bear Behelits and reach their lowest point of despair into demons called Apostles via a ceremony that requires the sacrifice of that which the human holds most dear to their heart...
...Slan was one of five humans who were transformed into godlike demons by means of the Crimson Behelit, a special Behelit that appears once every 216 years...
Apostles (Berserk) can turn humans into demons by infesting them with their demonic essence or infusing them with demonic power, turning them into Apostle Spawn. Mozgus, pictured above, is a particularly powerful Apostle Spawn.
D'Hoffryn (Buffy the Vampire Slayer) can transform humans into Vengeance Demons.
The Order of the Sword (Devil May Cry 4) use the Ascension Ceremony to turn their members into angelic-looking demons...
...as shown here with Sanctus.
The Dragon Eater (Dog Days) can corrupt and transform any living creature, including true dragons, into demons via its tendrils.
Those who drink the blood of Muzan Kibutsuji (Demon Slayer: Kimetsu no Yaiba)...
...will become a demon.
Jackal's Etherious Form (Fairy Tail).
Doriate's Demon Form (Fairy Tail).
Franmalth's Etherious Form (Fairy Tail).
Ezel's Etherious Form (Fairy Tail) Slash Attack Mode.
Kyouka's (Fairy Tail) Etherious Form.
Torafusa's (Fairy Tail) Etherious Forms.
Tempesta's (Fairy Tail) Etherious Forms.
Marde Geer (Fairy Tail) in his Etherious Form.
Zeref Dragneel (Fairy Tail) resurrected his dead brother Natsu as his strongest Etherious, E.N.D.
Evil Pieces (Highschool DxD) can turn other beings into devils.
Asia Argento (Highschool DxD) being demonized by an Evil Piece.
The Hood (Marvel Comics) could sometimes assume a demonic form...
...by giving himself fully to the powers of Dormammu.
When its bearer dies, the Mark of Cain (Supernatural) resurrects them, while transforming their soul into a demon...
...such as Dean Winchester...
As the result of her Book of Demons being filled, Bookend (Valkyrie Crusade) became a demon herself.
Ye Shen (The X-Dormitory), as the Embodiment of Evil...
... can demonize and corrupt benevolent power-users...
... by fighting his brother, the Embodiment of Goodness.
The Shadow Sword (Yu Yu Hakusho) can convert any human it cuts into a demon.