User becomes stronger, faster, more durable, etc. from the act of cannibalism: eating the flesh of a member of one's own species, like a human eating another human, possibly unlocking abilities related to the affinity and enhancing their existing powers. Some users may be able to draw sustenance from the act or even slow or stop aging.
Aaroniero Arruruerie (Bleach) gains strength by cannibalizing on other Hollows, but he is unique even among that as he also gains all the abilities of those he devours.
Szayelaporro Granz (Bleach) healing his wounds by eating his modified Hollows.
Level 3 Akuma (D.Gray-man) are not above devouring fellow Akuma below their levels.
Aldrich, Devourer of Gods (Dark Souls III) was a cleric that developed an appetite for his fellow man. This eventually led to him becoming so bloated and grotesque that he became a monstrous sludge-like creature.
Everyman (DC Comics) can change his form by eating the parts of other humans.
Wendigo (Native American Folklore)
When a Rugaru (Supernatural) tastes human flesh, they complete their metamorphosis from human to monster.
By repeatedly cannibalizing other ghouls, Ken Kaneki (Tokyo Ghoul) became more powerful until he manifested a Kakuja, a secondary kagune that acts as body armor.
Wendigos (Until Dawn) are a supernatural race of monsters that manifest when a human commits cannibalism on Blackwood Mountain due to the presence of the malevolent spirit Makkapitew.
Billy Bates (Until Dawn) was forced to eat the bodies of dead miners to survive being trapped in the mines, turning him into a Wendigo.
The Makkapitew (Until Dawn) is a malevolent spirit that haunts Blackwood Mountain and is responsible for the existence of the Wendigos that prowl its forests.
After nearly falling to her death off of a cliff, Hannah Washington (Until Dawn) was forced to eat her sister Beth's corpse to survive...
...forcing her transformation into a Wendigo.
Josh Washington (Until Dawn) becoming a wendigo.
Tōkajin (InuYasha) ate humans who showed even the slightest spiritual powers to enhance his own.
Momoshiki Otsutsuki (Naruto) ate his father Kinshiki to gain a power boost.
Charlotte Linlin (One Piece) accidentally devoured Carmel alive as a child...
...inheriting Carmel's Soru Soru no Mi in the process.