Old gods

When a god loses all of their followers and is forgotten, unless they are an Elder god, they lose their divine powers and godhood. This causes their bodies to decay and transform into hideous monstrosities physically. When a god loses their godhood, no matter how many new followers they have, they will never be able to regain their divine strength.
Ironically there are small people groups that worship the old gods after they have become an old god. Several tribes and witches pay homage to them and some even call upon them as a patron for warlock powers.
When a god becomes an old god, not all power is striped from them. They still maintain immense power of whatever they were a god of from before. They just lose their divine right and god status. There are some old gods whose pure power alone can rival that of a god.
If you choose to worship an old god you might get a more intimate experience with them than with a traditional active god of one of the many pantheons. Old gods cannot acend to the celestial plane like before and are forced mainly to walk among the mortals in the material plane.