The Descent of Eos

The elves of The Veil worshiped the Red Moon, Eos, whom they believed to be the celestial embodiment of their patron goddess of the same name. By her light, the elves were guided to the afterlife once they reached the end of their time on the Material Plane.
As the most elegant and powerful of all celestials, Eos was betrothed to the brave Titan Raxxis, Defender of the Realm. Raxxis was given the task of protecting the Material Plane from neighboring planes.
[Evil entity] sought to corrupt the Material Plane with darkness, and crafted a gloomy, inverted aspect of the Material Plane that consisted of a vast network of ominous caverns and tunnels. These caverns were roamed by aberrant creatures whose very existence are fueled by the maddening darkness that surrounds them. [Evil entity] stole the Maze Engine (aka the Orderer), a primordial mechanism that kept order between planes. He used this machine to merge his dark plane with the Material Plane, thereby swallowing the plane in his chaotic darkness.
His plan was successful, and opened a great chasm on the Material Plane. From this chasm spilled forth abhorrent darkness and nightmarish horrors that had thus far been unknown on this plane.
In the ultimate sacrifice, Raxxis gave his life, using his primordial form to seal the dark rift. In doing so he banishing the dark denizens to live below ground in a world that had since become known as the Underdark.
Eos, the Red Moon, was wounded with unknowable grief at the death of her betrothed. Her celestial form became heavy with sadness, and she plummeted to the world below.
She landed on the Material Plane, her impact creating a great mountain. Here she lay dormant in grief, until she was awakened in The Darkening.
With their goddess in mourning, the elves had no light to guide them to the afterlife. For the first time, their elders died of old age, their wisdom and knowledge lost forever.
The elves now live in the Veil, guarding their fallen goddess with great loyalty, awaiting the day that she may rise again and guide the souls of their ancestors into the afterlife.