Villains that are not physical beings but rather exist in the astral realm (though they may be able to possess others). Alternatively, they may not be real beings at all but psychic energy given life (or the illusion of life) by some powerful force. Sometimes they are simply created inside a real character's mind and only exist within their heads.

From imaginary friends (like Tyler Durden) to beings that act as people's nightmares (such as The Grand Duke of Owls), these beings, while not real creatures can create a strong mental challenge by making real characters and possibly the audience unable to tell the difference between what's real and what's not real.

Furthermore, Thought-Forms can also include actual villains who appear in the hallucinations and dreams of others, good examples being Slade and Bill Cipher.