The Demon Mask was an ancient and bizarre artefact with supposedly unnatural powers that are tied to Demons.
It was horrific in design; resembling a common perception of a demon with large bulging eyes, two long horns, and sharpened teeth. It appeared to be crystalline until it was later revealed to be plaster and luminescent paint.
The Demon Mask was a mysterious glowing artifact discovered by Professor Asuka, the "Father" of Ryo Asuka. After trying on the mask, Professor Asuka goes insane and is partly possessed by a demon. He kills Ryo's canary and his pet dog John and then attempts to kill Ryo, only for his human side to reemerge and leading Professor Asuka to commit suicide.
Sometime after his father's death, Ryo invites his friend Akira Fudo around to his home and tells him to put on the mask so he could understand about the origins of demons and how they are escaping back into the human world. He does so and is bombarded by visions of demons in the ancient past.
When Ryo starts to have doubts about his life and desires to become a devilman so that he may aid Akira in his fight against the demons. But when putting on the mask it doesn't seem to work anymore. Frustrated, he throws the mask onto the ground, whereupon it shatters and Ryo discovers that it was actually made of plaster with a coat of phosphorescent paint. Ryo finds out that he is not the real Ryo Asuka, who died in a car crash a long time ago and when Psycho Jenny shows up, he restores Ryo's memory and he realizes his true life as Satan.
It is implied that Psycho Jenny made the mask and it was her powers that showed Akira, Professor Asuka, and Ryo the visions and that the mask was nothing more than a tool.