They believe there were once Three Goddesses who watched over their world. the same way other Gods looked over their own worlds.
But one planet's God was arrogant, and considered it's planet the most beautiful of worlds because it glowed a bright light, all other worlds revolved around it, basking in it's warmth. The God continued to fuel this glow. Growing it bigger and brighter until eventually the planet cracked.
The Planet split like an egg, giant tendrils of light emerged from the planets shell, painted red by the blood of the planets life.
the tendrils went from planet to planet.
Corrupting the planets to create more tendrils to corrupt more worlds.
It spread through existence. Consuming life and Gods alike.
These Tendrils were on their way to Sanctum and the Goddesses knew it. So they decided to hide Sanctum away. Sacrificing themselves in different ways.
One Goddess; the Jest gave her Skin to create drape To hide Sanctum from the Light. This is The Veil
The Second Goddess; The Fair gave her Quills to create her own Green Tendrils on Sanctum, to fight the Red Tendrils from the light and to keep the Bound hidden. These became the Vines
The Third Goddess; The Scholar gave the biggest sacrifice, she sacrificed her very immortality to be born among the Bound and to guide them into the Vinefields and keep them safe from the light. She became The Priestess who still guides the Bound today.
The Bound call themselves the Bound because they are Bound to this world, their Sanctum, the only life left in the Universe.