fanfiction and derivative works — am I screwed?

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Illustrative scenario prepared by my practice.
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Hypothetical question

Has anyone dealt with something like this? I'm not sure what my options are lol.

fanfiction and derivative works. I've been dealing with this for about 12 weeks now and the situation isn't improving.

This involves software/code I developed over the past 6 months. I do have documentation proving my ownership and timeline.

Should I hire a lawyer for this or try to handle it myself?