Fan art, fanfic, and fan video are delightful passions, and like all such things, if they go too far, someone might get angry. DeviantArt and the Organization for Transformative Works, together holding the largest collection of fanworks in the universe based on any intellectual property within any media, will bring out their lawyers to explain how you can go to sleep at night, dream the dream of fans, and never have to hide under the bed.
