Some adventurers are content to be earthly rulers, basking in the rewards of their past glories. But some yearn for a further challenge and the greatest of all rewards.
Like concentric spheres, the higher planes surround the Prime Material. Beyond them all is the Empyrean, where dwells the Increate Source. This supernal presence is said to bestow godhood on those who reach it.
Getting there is the hard part. The paths are hidden in the lower astral, where there are monsters, godlings, devils, and beings on the same quest to get in the way.
Beyond the astral are the ascending levels of the Outer Planes, iconic realms ruled by (or perhaps manifesting) gods. Each is a challenge, perhaps designed to cause seekers to falter and fail, and possibly even be cast into the Abyss for their audacity.
All of reality is a mega-dungeon that goes up.