This is a myth I made up, not one I have heard before. I did, however, base these beings in the story off of the Taltos from the Anne Rice novels.
Once upon a time, the Earth was peopled by immortal beings. There was no sickness and there was no death. They were extremely tall, averaging seven feet, with long legs, arms and fingers. Perfect for performing the magic that was within each of them. They lived in tribes around what is now Europe, but were all connected telepathically. There were no strangers anywhere.
But there were, unfortunately, some evil beings among them. When evil was done, the council of elders would gather together, in what is now called Scotland, to judge the punishment of the offender. Usually these punishments were mild for an immortal being, a century of banishment from their tribe, public shaming, or in extreme circumstances, imprisonment.
As time went on, the beings all began to ignore the council. They felt as immortal beings they didn’t need to be ruled and they could do as they pleased. They became more brave and daring with their breaking of the rules. They ignored punishments shunned the council all together.
This enraged the council. They had always been a peaceful people before, and now all hope of that was lost. So the council made a life altering decision, they would strip the immortality from the beings, including themselves.
The beings were furious, war broke out between the ones that followed the council and ones that did not. For the first time ever, an immortal being died. Followed by another and another and another. Eventually, the only beings that were left were the council of elders, who had gone into hiding.
As the council grew old, and knew their death was inevitable, they pooled all their magic to create a smaller race of people. These creatures, which they called humans, would life their lives, procreate, and die within the course of a few decades. They children would be born small, then grow to their normal adult selves, then they would die as new humans were born. They called this the circle of life.
Eons ago it was proven that immortality was more of a curse than a gift. This why we are born and why we die.