The feywild, mortal plane and shadowfell form a path which every mortal soul travels. New souls enter the world through the ley nexus in the feywild and, when they shuffle off their mortal coil, exit through the ley nexus of the shadowfell.
Long ago, a fearsome dragon named Monatiss tried to take the power of the shadowfell’s ley nexus for himself. He succeeded, and for a short time death was no longer a threat, until the gods imbued a great hero with the strength to stop Monatiss.
Monatiss couldn’t be killed, for it was linked with the ley nexus and doing so would shatter it. Instead, Monatiss was returned to the location of the ley nexus at the foot of the Penumbra Cliffs, along the shores Sea of Despair in a massive cavern barely able to contain the ancient dragon.
There, for countless millenia he has remained, bound by enchanted iron chains forged by Moradin himself. Each link of the chains is as wide as a human is tall and weighs more than the greatest tree in the feywild. The dragon physically reunited with the ley nexus, its function was restored.
And so now, when a mortal dies, its soul passed through to the shadowfell, flies across the Sea of Despair, and enters the astral sea through Monatiss.