Lightstar Crystal (Skeleton Warriors); transforms humans into Skeletons
Tribal Mask (My Babysitter's a Vampire); transforms whoever with a costume into an actual character
Invincishroom (Mario & Luigi: Superstar Saga)
Mortal Cup (The Mortal Instruments); transforms humans into Shadowhunters
Infernal Cup (The Mortal Instruments); transforms Shadowhunters into Endarkened
Genie Lamp (Once Upon a Time in Wonderland)
Goruda Brain Tonic (Doctor Who)
Immortality Gate (Doctor Who)
Butterfly/Moth Miraculous (Miraculous Ladybug)
Pontiff's Left Eye (Dark Souls III)
Old Blood (Bloodborne)
Pearl of Transformation (My Little Pony: Friendship is Magic)
Electro Axe (DC Extended Universe)
Moon Pool (H2O: Just Add Water & Mako Mermaids); transforms humans into merpeople
Okami Mountain Spring (Majimoji Rurumo); transforms humans into wolves
Planet Geo (Guy: Double Target)
Deacon Frost (Blade) viewed Vampirism as a gift.
The Shroobs (Mario & Luigi: Partners in Time) convert other beings into Shroobs or Shroob-like beings via a process called "Shroobification."
Diagon (Ben 10) could convert countless beings into Esoterica, his cross dimensional foot soldiers.
Joseph Bertrand III (Infamous) converting a human into a Corrupted.
The Moon Pool (H2O: Just Add Water and Mako Mermaids) transforms any human into merpeople on a full moon.
Cybermen (Doctor Who) seek to convert every living thing in the universe into one of their race
Doctor Constantine (Doctor Who) being transformed into a gas mask zombie
Noah (Doctor Who) being transformed into a wirrn.
Using the Immortality Gate, The Master (Doctor Who: The End of Time) turned every human on earth into 6,727,949,338 clones of himself.
Saturnyns (Doctor Who) could covert other species into members of their own kind via blood transfusion
Children being converted into Axons (Doctor Who: The Golden Ones)
Hawk Moth (Miraculous Ladybug), with the power of the Butterfly/Moth Miraculous, is able to transform others into supervillains, complete with their own arsenal of superpowers.
Darkblade (Miraculous Ladybug) has the power to convert humans into members of his army of knights.
Pharoah (Miraculous Ladybug) is able to channel the power of Anubis, converting humans around him into mummies.
Puppeteer (Miraculous Ladybug) has the power to control those who's caracature she possesses, using Marinette's dolls as a means of doing so. When using this power on purified supervillains, the villains regain their powers and villain identities.
Gamer (Miraculous Ladybug) is able to convert humans into units of power, upgrading his mech with them.
Lilim (Monster Girl encyclopedia) Are an enhanced race of succubus who are hailed as daughters of the Demon Lord; like their weaker counterparts, they have the ability to transform others, be they male or female, into different kinds of monster, demons or beasts.
When her elemental powers reached its peak, Princess Bubblegum (Adventure Time) was able to turn everyone and everything around her into candy, as well as make them more sweet in behavior. She was also shown to be able to convert even other Elementals.
When her elemental powers reached its peak, Patience St. Pim (Adventure Time) was able to turn everyone and everything around her into ice, as well as make them more depressed in behavior.
When her elemental powers reached its peak, Slime Princess (Adventure Time) was able to turn everyone and everything around her into slime, as well as make them more relaxed and festive in behavior.
When her elemental powers reached its peak, Flame Princess (Adventure Time) was able to turn everyone and everything around her into fire, as well as make them more enraged and blood-hungry in behavior.
Pontiff's Left Eye (Dark Souls III) is a type of ring created by Pontiff Sulyvahn to be given to his Outrider Knights, corrupting them and turning them into frenzied beasts...
...such as the unnamed Boreal Outrider Knight...
...Vordt of the Boreal Valley...
...and the Dancer of the Boreal Valley.
The Succubi (Monster Girl Encyclopedia) are hailed both by other mamono and the Demon Lord as the ultimate monster for their ability to procreate more and more monster girls, even monster boys who they take as their mates.
The energy emitted by Steppenwolf's (DC Extended Universe) Electro Axe can convert humans into Parademons.