Users become stronger, faster, more durable, etc. when they come in contact with evil and malice, including of oneself and others, possibly unlocking abilities related to the affinity and enhancing the existing powers. Some users may be able draw sustenance from evil or even slow or stop aging.
User's power is limited to the amount of negative energy or intent in the environment
May lose power if placed in an environment with no malicious energy (i.e. a Buddhist Monestary)
Lavos (Chrono Trigger); via Evil Emanation
Fanoxean (The Young Guardians)
Chaos Gods (Warhammer 40000)
Avenger/Angra Mainyu (Nasuverse)
Trigon (DC Comics)
Malachite (Steven Universe)
Janemba (Dragonball Z)
Mah-Oh (Granstream Saga)
Vice ( Karakuri Dôji Ultimo)
Noob Saibot (Mortal Kombat)
Garuda (Street Fighter series)
Mephisto (Marvel Comics)
Spawn (Image Comics)
Black Lily (Valkyrie Crusade)
Orb of Caldia (Marchen Awakens Romance)
Spike (Dragon Ball)
Null (Teenage Mutant Ninja Turtles Adventures)
Black Lily's (Valkyrie Crusade) power is her own dark malice, so the more malice she has, more powerful she becomes.
Spike's (Dragon Ball) Devilmite Beam, amplifies any dark thought (anger, malice, aggression, arrogance, etc.) and converts it into pure negative energy, which expands in the victim's body until their heart explodes.
Being a creature made completely out of evil energy, Orb (Marchen Awakens Romance) was able to absorb the darkness of human life to turn into a vile form. The ugliness of this form is the evil side of humans' hearts.
Evil Stewie (Family Guy)
Null (Teenage Mutant Ninja Turtles Adventures) becomes more demonic and gains new powers with every evil deed he commits...