Our campaign world is kind of a Planescape-ish Multiverse. With a lot of influence from some of the lore of the old Planescape, a pinch of every other edition of D&D (even 4e), and a huge dose of Pratchett, most of the gods in our setting can be considered Tulpas, but the narrative we created goes something like this:
In the beginning the 3 Progenitors (Ancient Brethren, Elder Titans, their names are lost to time) found a way to create a new universe beyond their own "Far Realm" universe. Their new young physical universe was simple at first: They had created the Elemental Planes. By experimenting with these Elements, they managed to then give birth to the Prime Material Plane. As life evolved (largely on its own) on these Prime worlds, they began to dream and imagine and create.
Belief was formed. Belief gave rise to the concepts of gods, afterlives, and ultimately the Outer Planes.
The Progenitors had mixed feelings about it. One ultimately decided the experiment had gone too far, and sought to unmake all that had been made (which gives rise to "Tharizdun / Khyber" type end-of-the-universe type destroyer legends). Long story short, we get our early gods against the Destroyer's Primordial minions, with 1 Progenitor choosing to remain neutral and side with the Primes (giving rise to the dragons as his guardians) and the other retreating into the Outer Planes and into obscurity. Our Destroyer gets banished, our Neutral Protector "dies"(?) and the third Progenitor is lost. So all 3 of the original beings are out of play now.
The winning gods get to define the rules of the universe and inspire various legends and stories in their mortal followers that suggest it was
they who created everything. Ideas of any Progenitors before the gods are heretical.
The currency of the gods is
Belief. With enough of it they can create miracles, without it they can shrink, wither, and ultimately die.
The gods are also beings who take on many faces and aspects, so a single god might go by a dozen different names and appearances across many worlds, though these will all line up with a certain theme (Death, healing, sun, et al).
Powerful mortals who can accumulate enough Belief can achieve demigod-hood and then ultimately true godhood if they persist and know enough about how it all works. Gods can also invest in mortal to create 'proxies' and 'exarchs' who can act like demigods on their behalf. Some powerful beings have also shied from becoming true gods, understanding that once their existence is tied to Belief they become forever dominated by it.
TLDR;
- 3 Progenitors (who are no longer players)
- Mostly Tulpas
- A dash of Apotheosis