Demon in Your Head

The "Demon in Your Head" was the name given to a villain found within a short horror story in Uncanny Tales of 1953, published by Marvel Comics long before they would become recognized as a shared universe of superhero fantasy: in this era the company focused largely on short stories in the style of Tales From The Crypt, Creepshow or The Twilight Zone.

Biography
The Demon in Your Head was a symbolic entity, representing madness; victims of its possession become unable to sleep, think clearly or breathe well, eventually they became completely insane and could do nothing but run in terror.

Throughout the story this malevolent spirit haunted numerous people, driving them insane before possessing another victim via jumping into their mouth, due to its magical nature the demon was completely invisible to anyone save for those it had already possessed.

The Demon in Your Head succeeded in traveling from Canada to the United States via one such host and ultimately hopped from them into the psychiatrist, who had believed that they had "cured" the madness, only for it to be revealed that the Demon was now active inside him and presumably getting ready to repeat the cycle endlessly.