
Picture found at arthistoryarchive.com
The Shadow is the dark part of us all and is the unconcious part of the mind. It is where our primal instincts are held.
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There are times where the shadow can take near complete control of your being and you are helpless to do something. There are cases where it's a good thing (read post before this one), but there are times where there is nothing good about it and it destroys your relationships. A great example of this is in Spiderman 3. After getting everything he wants tanks to the symbiote is telling him what to do and say, he does something that the symbiote tells him to do (which is hitting Mary Jane watch the movie if you want info) he tries to get rid of it. He was sucussful but he created a new monster.
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Venom is the remainder of the symbiote (shadow) merged with Eddie Brock (who let it take over) and used it to be evil and get revenge on Spider-man for ruining his life and such.