User with this ability either is or can transform into mask or masks, of any material. Users' transformed form is either anatomically identical to their normal form, aside of being made of masks, in which case it contains all to organs and is somewhat vulnerable to attacks. Alternately the user can transform into homogeneous matter, without any part of their form being more important than the other.
Hata no Kokoro (Touhou Project) Expressionless and Emotional Menreiki.
Tarakudo (Jackie Chan Adventures) is an all-powerful oni that has taken the form of a giant, floating mask.
On more than one occasion, Hexadecimal (Reboot) is shown with the power to shift between a full body and her disembodied mask for traveling purposes.
The Loas (The Princess and the Frog)
Orche (March Story) became possessed by an Il - a dark spirit spawned from works of art - that inhabited a mask, being granted various powers associated with it at the cost of losing all of his free will to the mask's spirit.
The Mask of Dark Earth (Sly Cooper) is an evil aboriginal spirit in the form of a mask that latches itself onto the nearest host, increasing their size and making them very aggressive
The Terror Mask (Splatterhouse) is an ancient relic of unspeakable power, granting its wearer superstrength, various gory abilities and guides the weilder with snark.
Aku Aku (Crash Bandicoot series) is a witch doctor whose spirit now lives in a wooden mask who guides Crash and his friends as they defeat the forces of evil.
Uka Uka (Crash Bandicoot series) is Aku Aku's evil brother.
The Elementals (Crash Bandicoot series) are a group of evil masks who have the power to manipulate the elements of earth, water, fire, and air and use them to cause chaos across the world.
SCP-035 (SCP Foundation) is an evil entity that takes the form of a mask who's expression alternates between "drama" and "comedy".