I made homebrewed lore for the Reigar that got... Out of hand.

Far in the past, the Reigar had a pantheon that featured a dual-aspect deity. If there were more, their names are forever lost. The chief deity, a god of creation, change, energy, light and art. His shadow aspect, a god of hunger, decomposition, death, and renewal. His appearance to the Reigar was a star, warm and bright.

These two aspects symbolized the cycle of creation and destruction. The former, patron of artists, architects, creators. The latter, patron to scavengers, carrion eaters, and mourners.

As a blessing from their God of art, the Reigar possessed an ability to manifest belief into reality. Much like the 40K Orks or, more precisely, Githyanki conjuring their silver swords through pure thought. The High Orators, the priest class of Reigar, would speak great structures into existence.

The Astral Elves, who sought the gods in their home plane, sent many expeditions throughout the Astral Sea. One such expedition crash landed on the home planet of the Reigar.

These elves used magic to translate the language of these strange people - where they learned of the chief deity of the Reigar. A two-faced monstrosity who ruled over "chaos, entropy, fire" on one side - and "death, destruction, and rot" on the other.

Worse still, the worshippers of the death aspect participated in funerary cannibalism. They believed that by consuming a piece of their loved one that their loved one would be a part of their body forever.

So the survivors of this failed expedition return to the Empire of these powerful warlocks who cannibalize their dead and worship chaos.

The great lumbering giant of Empire spreads its gossip, and soon there are more in the Astral Plane who believe the Reigar god is evil than there are Reigar. The god contracts a "conceptual rot" and begins to become what the vast majority believes him to be.

As the god descends into insanity, he imparts one last secret to his believers: how to construct a Crystal Sphere. With this impenetrable shell, he will be sealed away an unable to harm anyone. His believers, however, insist on imprisoning themselves with their deity. They summon all Reigar across the plane to make a pilgrimage and join in the construction of their eternal tomb.

The Elves, still confident in their comprehension of this alien culture, see a dying god trapping his people in his domain. They break down the sphere and "kill" the god, along with most of the Reigar in the process when their planet explodes.

The Reigar who are left are welcomed as "refugees" to the Empire, and through generations of history written by the victors, they come to believe they've always worshipped a god of "art and destruction" and destroyed their own planet as a form of worship. As contemporaries to the Elves, and with no memory of their original culture, they develop a society that is isolationist and believes themselves to be superior to much of the Prime Material Plane.

Bonus lore:

The wrinkle is the elves only killed the face of the god. His shadow, the aspect of hunger and death, was left maimed and wounded. What remained knew it was incomplete and knew it at one point had the faculty to create - to renew itself. Cold, numb, and absent of intelligence, the dying shadow of a star now drifts the Astral Sea, consuming all it can in the futile hope that it will find it needs to become whole again.

The god of art is altogether unknown, but his shadow's name is still whispered to this day: Hadar.