One day, a bored primal goddess, decided to have some fun and made a stone that transforms people based on certain conditions. She made it to look like an unassuming cube of rock and hid it in a verry hard to reach spot.
Over the time, since no one managed to find the cube, it was forgotten by everyone except her. Even if she didn't mind what would happen if the cube will be found by someone, she liked to see how good will the cube fare out in the wild.
Time tough is a verry tricky thing and put the cube on a weird journey.
It first made that the hiding place become buried and forgotten, then it slowly made that the rock around the cube fade away and bury the cube further in the depths of the earth. At one point, the cube was buried so deep, it started to change how it looked, and skewed itself into a fat rectangle with some cracks and chipped edges. Some more time after that, it started to reach up towards the surface, using volcanos and magma, thus ending up a little glassy and melted. Next, it ended up on a bed of limestone and, over the time fell in the cracks that naturally form there, only to end up, after a relatively small colapse in a cave, in an underground river rapids. The colapse broke it in half and crushed one of those halves into fine dust, the other half being carried out by the rapids and shaped in more or less the current shape of it, an oblong oval that is slightly fat, of course this was due to the force of said water, but it did not help the fact that after a long journey underground the river met the ocean and made some other natural changes to our stone that now lay down on a pebble filled beach somewhere in the world awaiting it's first victim