A Thousand Years.

It is said that once you die, your soul is ferried across the river Styx, and as you stroll along the floral shores of the rivers of Hades, you will reach a bridge over the last river. A spirit guarding the bridge named Lethe awaits your arrival to offer you water from her river, mixed with some herbs along her shores to drink, before she may grant you passage across the bridge towards the gate of rebirth.

The effects of drinking this tincture will allow you to forget everyone and everything; all the love and all the hatred, all the joy and all the pain; to begin anew in the next life.

Yet, there are some who refused the tincture to forget for personal reasons. As passage over the bridge cannot be granted without, Lethe took pity on these souls, and leaves a distinctive mark on their bodies in the form of a dimple on their cheek.

She offered a different way of passage: if one is to pass without the potion of Lethe, they must bathe in her river and survive the burn of its ice and fire for a thousand years, before they can reach the shores on other side; and only then they will be allowed to walk through the gate of rebirth with full memories of their past lives intact, so they may yet find those they once loved.

So, be kind to dimpled cheeks among you, whether kin or stranger. You may never know who they were to you in your past lives, and how much you meant to them, so much so that they are willing to suffer in wait for a thousand years just to find you again; perhaps for a promise he or she has yet to fulfill. Please do not ever hurt them, because not everyone is brave enough, like they were, to endure a thousand years of torment to swim across river Lethe, just for the chance to spend another lifetime with their loved ones again.

This is an English interpretation of the concept from Chinese mythology that has not been told in other languages nor found in other mythology.