Some questions about it:

• If I create a funny version of a copyrighted game and sell it (crediting the original developers) would it be considered a parody or would I need to pay royalties?
• If I create a game mod for my own personal use would it be legal? What if I gave it for free to a friend?
• Is there a general rule about it or it depends on the developer will?

P.S.: I'm not talking about cloning games like this question. It's all about a game clearly based on another. Something like "GTA Gotham City" ;)

EDIT: This picture that I found over the internet illustrate what I'm talking about:

Just in case I was not clear:

• I never created a mod game. I was just wondering if it would be legally possible before trying to do it.
• I'm not apologizing piracy. I pay dearly for my games (you guys have no idea how expensive games are in Brazil due to taxes).
• Once more I say that the question is not about cloning. Cloning is copy something and try to make your version look like a brand new product. Mods are intended to make reference to one or more of its source. I'm not sure if it can be done legally (if I knew I wasn't asking) but I'm sure this question is not a duplicate. Even so, I trust in the moderators and if they close my question I will not be offended - at least I had an opportunity to explain myself and got 1 good answer (by the time I write this, maybe some more will be given later).