The Haunting Shadow In Your Dreams

This is an excerpt from Ariadne Green's book, "Ariadne's Book of Dreams : A Dictionary of Ancient and Contemporary Symbols" from enotalone:

A powerful archetype, the shadow, perhaps the most disowned and denied power within the subconscious, may appear out of nowhere to threaten the goodwill of the dreamer. It may appear as a villain, an assassin, a tyrant, or even as the Devil himself to torment and threaten our sense of worth or fulfillment. Like the Grinch (who stole Christmas) or the Wicked Witch of the West, the shadow may indicate the power of evil, which must be confronted and challenged in order to win back our innocence and embrace fulfillment. The nightmarish quality of a shadow dream may bring about fear and terror. Most of us have had a confrontation with the shadow in a dream, in which we are being chased, threatened, or robbed of actualization. We may run or hide when he pursues us and awaken with a jolt. The shadow rests in our subconscious, the place where we may deny the evil within ourselves in order to live moral lives. Our shadow nightmares reveal this conflict within ourselves.

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Our shadows cannot be denied, bargained with, or destroyed. They must be faced, or their evil will continue to haunt our dreams and rob us of our opportunities for fulfillment and pleasure in life.

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Those within the culture who are attracted to negative symbolism, who wear tattoos of devils and images of death, may act out the disowned shadow for the culture as a whole. Positive or negative overidentification with archetypes may be dangerous because this assigning of black or white devalues other pieces of the self that, when integrated, offer a more complete picture of the individual.

DreamTherapyNow has a little more on those shadow pieces in your dreams: 

Working with your shadow is a term which describes becoming aware of the dark side of ourselves.  This does not mean that it is bad, but something we reject and fear. Our Shadow side must be recognized as something both "other" or different from what we want to be and something that is needed to become whole.  Most often, when Shadows appear to us in dreams, they hold the key to our repressed memories, which enable us to uncover unwanted habits and allow us to heal.   For example a shadow will come to us most often as someone of the same sex in dreams.  He/She will be our old childhood friend or co-worker who is completely opposite from us; they are incompetent, loud, rude, obnoxious, sexual or docile.  The traits they carry which we reject as bad or "other" (not of us) are actually traits which we fear or need within ourselves; they can also be traits which we show outwardly to our family and friends but are not aware of it. Traits which hurt our relationships and ourselves.  This can also be called Projection: when we project onto others what we deny as part of ourselves.  (For example accusing your spouse/partner of not being more supportive when in fact it's you who needs to show more support.  Or, wondering why he/she doesn't love you more when in fact you don't love yourself enough or give yourself more worth.)   I am not by any means saying that if you dream of a bankrobber or rapist that this is something you need to do to become whole! Rather, someone who dreams of a rapist should look at how they treat the opposite sex...abusing power over anyone, something we may not be aware of, can come out in dreams of this sort. They can also reveal past abuses that we may be repressing.