Mumbo Jumbo (Banjo Kazooie)
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.
Apostles like Mozgus (Berserk) can also turn humans into demons through infesting them with their demonic essence or infusing them with demonic power, turning them into Apostle Spawn.
La Magra (Blade) can turn humans into vampires by merely walking past them.
D'Hoffryn (Buffyverse) can transform humans into Vengeance Demons.
Zobek (Castlevania: Lords of Shadow) can transform the dead into necromancers through the power granted by Satan.
Evil Ernie (Chaos! Comics) can turn dead into Dead Onez with his arcane energy.
Athena (Greco-Roman Mythology) could turn humans into hideous monsters.
Pontiff's Left Eye (Dark Souls III) is a type of ring created by Pontiff Sulyvahn to be given to his Outrider Knights, transforming them into frenzied beasts.
Ahmanet (Dark Universe) was able to suck the life out of others, rejuvenating herself and mummifying them in an instant, turning them into her undead slaves.
Zatanna Zatara (DC Comics) has been known to transform others from one species to another, or even into inanimate objects with her magic on occasion.
Madame Xanadu (DC Comics)
Enchantress (DC Extended Universe) can transform humans into Eyes of the Adversary.
The Order of the Sword (Devil May Cry 4) use the Ascension Ceremony to turn their members into angelic-looking demons.
Molag Bal (The Elder Scrolls series) created vampirism by raping Lamae Beolfag, cursing her with unlife in the process.
fal'Cie (Final Fantasy XIII) can give humans the brand of a l'Cie, granting them magical powers, but with the progressive development into either a crystalline form or a monstrous, mutated Cie'th.
As the leader of the White Walkers, The Night King (Game of Thrones) can raise vast armies of wights with minimal effort, and turning humans into White Walkers with his magic.
Kai (Kung Fu Panda 3) can use his control over chi to create Jombies.
Kain (Legacy of Kain; in the center) resurrected the Sarafan Inquisitors, the mightiest vampire hunters in Nosgoth's history, turning them into the greatest heralds of his vampiric empire.
Dormammu (Marvel Comics) can transform others into demonic entities with his control over magic.
Samuel Barone/Black Talon (Marvel Comics) uses voodoo to create zombies.
Sersi (Marvel Comics) has transformed people into animals through magic.
As a master of voodoo, LeChuck (Monkey Island series) can use it to turn others into undead.
As a master of necromancy, Quan Chi (Mortal Kombat) can turn the dead into revenants or even de-roboticize them to do so with his Netherrealm magik.
Shalltear Bloodfallen (Overlord Maruyama)
Calypso (Pirates of the Caribbean series) can use her magic to transform beings into different forms, such as reversing Hector Barbossa's undead state and turning Davy Jones into a monster.
Dr. Facilier (The Princess and the Frog) can turn others into frogs through his magic.
The Muur Talisman (Star Wars Legends) is a Sith artifact that transforms any sentient creature, with the exception of force-sensitive beings and a few alien species, into mindless Rakghouls through dark side alchemy.
When its bearer dies, the Mark of Cain (Supernatural) resurrects them, while transforming their soul into a demon....
...such as Dean Winchester...
As the Lich King, Arthas Menethil (World of Warcraft) could turn even the living directly into undead with his magic.