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.
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.
The Dragon Eater (Dog Days) can corrupt and transform any living creature, including true dragons, into demons via its tendrils.
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.
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.
The Hood (Marvel Comics) could sometimes assume a demonic form by giving himself fully to the powers of.....