: Can I reference events of a different fictional work in my fictional work? Let's say I'm working on a fantasy series called MyWork, that is set in the present in a small town in the US.

I think that you should ask yourself if you really need to do any of those four items in your story. Each of your items is a form of borrowing from another author's work. I'll ignore most of the legal stuff because the other answers explain it better than I can.

As has been stated in other answers, it's legal to do this if the work you are borrowing from is in the public domain. And some authors do this.

In the case that the borrowed works are not in the public domain, then you are opening yourself up to legal action, such as a lawsuit. This can be extremely expensive to defend against, even if you are in the right.

It is my personal opinion, but I don't like writing fanfic. I also don't enjoy buying a book and then finding out that the author has written something I cannot unexpectedly appreciate without reading (or watching) other works.

I'll try to go through your list and explain why I consider them dangerous. The main thing I find is that you have given control, to one degree or another, of your writing. My comments are mainly applicable in the case of works under copyright, however, even public domain stories can be written into new series (there are many such about Sherlock Holmes).

Mentioning that the events of IW take place in the same universe as
YourWork.

This can be dangerous because you have lost control of your setting. For example, let's say that your works are set in the city of New York. Then imagine that the author of the borrowed work releases a sequel in which he destroys New York in nuclear fire. This means the sequel you're writing will now have to be modified or tossed.

Have a character in YourWork (YW) reference events from IW.

This is a weaker version of your first point. It might even be legal if the events are close enough to the real world events that your readers don't notice. It still is giving some control to a third party.

Have a character in YW somehow in touch with character from IW,
without mentioning name of IW character.

To me this doesn't add anything to your work. If the reader can't figure out who the IW character is, then you run the risk of alienating that reader if he finds out. Essentially this could be thought of as laughing at the reader ("I know something you don't know!")

Have a character in YW somehow related to a character of IW, without
mentioning name of IW character.

Again, if the reader cannot figure it out on his own (in which case why bother to hide the name?), you're just laughing at the reader. And again, you've added nothing to your work if the reader doesn't know the relationship.

These are strictly my opinions. I don't write fanfic.

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In the case of copyrighted work, the critical question is whether you are referencing the other work or making use of it.

Simply referencing it is usually okay, especially if it has become a well known expression or meme:

punching a side of beef like Rocky
running up the Philadelphia Museum of Art steps
"Yo Adrian!"

But if you are actually making use of the other character or location directly in your story, that's almost certainly not okay:

you hire Rocky to help you with something
you visit the pet shop and ask Adrian for advice.

You might get away with it if you are parodying the other work, or if you are paying homage to it, but it had better be very obvious that you are doing so and not simply exploiting the other author's work for your own benefit.

Imagine if it were music instead. You might get away with incorporating a bar or two from someone else's tune as an obvious tribute to them, but simply using their work as if it were your own is outright plagiarism.

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It's tricky. It's very complicated I don't believe there is a definite answer. I see no reason why you cannot reference fictional worlds or characters - even by name. I see examples every day. The pop culture genre is filled with such references - some more subtle than others.

"The guy just hulked out and went all Rambo on the other guy's ass."

"It seems we are not in Kansas any more."

In the way that a substantial number of common expressions are derived from Shakespeare, pop-culture, indeed, modern language, is filled with terms and relationships from fictional worlds.

There are two main legal tests for your intended usage: (1) Profit: is your use of the relationship a contributory factor in the sales of your books. i.e. Did people buy your work because it was based on 'Hogwarts'? (2) Brand damage: By indicating your character is Batman's gay half-brother, have you damaged Bruce Wayne's reputation.

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First, please take this answer with a (big) grain of salt. I'm not a publishing or legal professional, so if you really want to know the right answer, you should ask someone who is. This answer is more just my two cents worth.

You have four scenarios where you possibly want to use InspirationWork (IW) in your own writing in some form. These are:

Mentioning that the events of IW take place in the same universe as YourWork.
Have a character in YourWork (YW) reference events from IW.
Have a character in YW somehow in touch with character from IW, without mentioning name of IW character.
Have a character in YW somehow related to a character of IW, without mentioning name of IW character.

Now, for the above situations, there are some questions I first want you to ask yourself which will greatly effect the answer of whether you can or cannot do this.

Is the InspirationWork in the public domain or otherwise NOT under copyright?

If the answer is yes, I believe you can do all of the above freely. All works and characters in the public domain are free to be used by writers other than the original author, whether for adaptations, sequels, spin-offs, etc.

But if the answer is no, then situations 1, 3, and 4 are a NO. Sure, you could mention them vaguely enough that if someone does sue you you can say 'But I was actually referencing this, not that!', but why risk it? You may as well create original interesting events to reference in your stories instead of making your writing a vague tie-in to already existing work.

As for situation 2 when it comes to non-public domain/still in copyright works, it's a bit of a grey area. If you mean mention IW as though it's part of the same universe as YW, then it's a no just like above. But, if you just mention it as a piece of fiction that is known in your universe, ('Hey, this reminds me of this thing that happened in a book I read ages ago!') then that can be OK. Sometimes. It depends on how clearly you reference it, whether you reference it in a good or bad light, the original author's temperment when it comes to their works being used/mentioned in other people's works, and I'm sure many other things. But like I said, I'm not a publishing or legal professional. But I would still be cautious.

Finally, though, if you have permission from the IW author, then there's no need to worry! You can do whatever they give you permission to do. But not more than that. If they give you permission to write a story with a character who's the cousin of their main character, but not permission to write a direct sequel, do not write a direct sequel. You would still have to be careful, though.

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