The user can cause things to change in form, character, or function, processing something to change from one form to another, permanently transform other beings into other kinds of creatures, typically the kind they are; for example, vampires transforming humans into other vampires through a bite. The methods used vary depending on the circumstances; generally through blood or bites, sometimes through injection or spells.
Pontiff's Left Eye and Pontiff's Right Eye (Dark Souls III)
Electro Axe (DC Extended Universe)
Goruda Brain Tonic (Doctor Who)
Immortality Gate (Doctor Who)
Invincishroom (Mario & Luigi: Superstar Saga)
M.U.L.C.H (Milo Murphy's Law: The Phineas and Ferb Effect); transforms humans into Pistachion
Butterfly/Moth Miraculous (Miraculous: Tales of Ladybug & Cat Noir)
Mortal Cup (The Mortal Instruments); transforms humans into Shadowhunters
Infernal Cup (The Mortal Instruments); transforms Shadowhunters into Endarkened
Tunguuk's Mask (My Babysitter's a Vampire); transforms whoever is in a costume into that character
Heart Stone (My Fairy Tail Love Story); transforms whoever touches it into a mermaid/merman
Pearl of Transformation (My Little Pony: Friendship is Magic)
God Tree (Naruto)
Genie Lamp (Once Upon a Time in Wonderland)
Doofenshmirtz's Repulse-inator (Phineas and Ferb: Night of the Living Pharmacist)
Silver Crystal (Sailor Moon)
Lightstar Crystal (Skeleton Warriors); transforms humans into Skeletons
The Muur Talisman (Star Wars Legends)
Magic Meat/Fofoo (Transformers Rescue Bots); transforms whoever consumes it into hairy beast during the night
Moon Pool (H2O: Just Add Water/Mako Mermaids); transforms humans into merpeople
Planet Geo (Guy: Double Target)
Okami Mountain Spring (Majimoji Rurumo); transforms humans into wolves
For fan-fictional to non-canon characters, see here.
Princess Bubblegum (Adventure Time) was able to turn everyone and everything around her into candy when her elemental powers reached their peak, as well as make them sweeter in behavior. She was also shown to be able to convert even other Elementals, like Flame Princess.
Patience St. Pim (Adventure Time) was able to turn everyone and everything around her into ice when her elemental powers reached their peak, as well as make them more depressed in behavior.
Slime Princess (Adventure Time) was able to turn everyone and everything around her into slime when her elemental powers reached their peak, as well as make them more relaxed and festive in behavior.
Flame Princess (Adventure Time) was able to turn everyone and everything around her into fire when her elemental powers reached their peak, as well as make them more enraged and blood-hungry in behavior
The Krylock Demon (American Dragon: Jake Long) venom is able to turn people or mystical creatures into Krylock-human hybrids
Skinner Sweets (American Vampire) converting Pearl Jones into an American Vampire by feeding her his blood.
Those with the power of the Founding Titan (Attack on Titan) can forcibly change Subjects of Ymir into Pure Titans, and presumably vice-versa.
Due to his royal heritage, but lacking the Founding Titan, Zeke Yeager (Attack on Titan) can also convert Subjects of Ymir into Pure Titans, but only after they have been exposed to his spinal fluid.
Diagon (Ben 10) could convert countless beings into Esoterica, his cross-dimensional foot soldiers.
Deacon Frost (Blade) viewed Vampirism as a gift.
The Hōgyoku's (Bleach) ability to manifest the desires and potential of any creature allows it to, amongst others, turn Hollows into Arrancar, Shinigami into Hollows, and humans into Fullbringers.
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...
...such as the unnamed Boreal Outrider Knight...
...Vordt of the Boreal Valley...
...and the Dancer of the Boreal Valley.
The energy emitted by Steppenwolf's (DC Extended Universe) Electro Axe can convert humans into Parademons.
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 kind via blood transfusion.
Children being converted into Axons (Doctor Who: The Golden Ones)
The Moon Pool (H2O: Just Add Water and Mako Mermaids) transforms any human into merpeople on a full moon.
Joseph Bertrand III (inFAMOUS 2) converting a human into a Corrupted.
The Shroobs (Mario & Luigi: Partners in Time) convert other beings into Shroobs or Shroob-like beings via a process called "Shroobification."
The Machine Used for Literally Converting Humans To Plants, aka M.U.L.C.H., (Milo Murphy's Law: The Phineas and Ferb Effect) was created by Derek to transform humans into Pistachions.
Hawk Moth (Miraculous: Tales of Ladybug & Cat Noir), with the power of the Butterfly/Moth Miraculous, can transform others into supervillains, complete with their arsenal of superpowers.
Darkblade (Miraculous: Tales of Ladybug & Cat Noir) has the power to convert humans into members of his army of knights.
Pharoah (Miraculous: Tales of Ladybug & Cat Noir) can channel the power of Anubis, converting humans around him into mummies.
Puppeteer (Miraculous: Tales of Ladybug & Cat Noir) has the power to control those whose caricature 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: Tales of Ladybug & Cat Noir) can convert humans into units of power, upgrading his mech with them.
The Succubi (Monster Girl Encyclopedia) are hailed by both 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 God Tree (Naruto) can convert any human ensnared in the Infinite Tsukuyomi into White Zetsu.
Lord Ravna (Star Wars) is a vampiric, batlike alien who can convert others into members of his species by scratching them.
The Muur Talisman (Star Wars Legends) is an ancient Sith artifact that allows its owner to transform any sentient creature, except force-sensitive beings and certain alien species, into mindless Rakghouls and control them as their obedient slaves...
...Celeste Morne using the Muur Tailsman to transform Darth Vader's Stormtroopers into Rakghouls.
With just swipe of his claws, Muzan Kibutsuji (Demon Slayer: Kimetsu no Yaiba) convert a human into a demon.
The Joker's (Batman: Arkham) blood injected five individuals that overrided their identities and, turning them into different versions of the Joker with specific aspects of his personality.