I am making a gnome warlock. I want his patron to be Cthulhu. You may know that warlocks get their powers from a patron. You may also know that in order for a warlock to keep his powers, he must do favors for his patron. At a point in his backstory, I want him to stand up to Cthulhu and revoke his patronship (if that’s a word). Would he still keep his powers? He would keep doing favors for Cthulhu if it were necessary.
There are no rules for this -- your DM will have to make a ruling. This will involve worldbuilding about the specific manner in which you receive your powers.
It would be dangerous for us to speculate too much about what the answer should be, because somebody might show that speculation to their DM and say: "well of course I can do this, the people on this Q&A site say it's legit!"
Some obvious answers would be:
- you lose all warlock magic
- you lose the bits of your warlock magic that are specifically tied to your otherworldly patron class feature (gained at 1st, 6th, 10th, and 14th level) but not the rest of it
- you keep your existing powers but lose the ability to gain further levels of warlock, because your patron has stopped sharing secrets with you
- you lose powers, but you can get them back if you find a different patron. (wise warlocks probably have the second contract lined up before they break the first one)
- your patron has some sort of story-based method to ensure your compliance, like it owns a piece of your soul or something, and breaking your patron bond involves a quest to get the piece of your soul back