The Story of Shaco
Once upon a time, there was a prince who was very lonely. His parents were busy with the business of running a kingdom, and the prince had no friends, and so a craftsman made him a companion: a perfectly crafted marionette, enchanted to be entertaining, to always be there, to always make the prince laugh. For a while, this worked. Then something changed in the marionette. What changed it, and why, is not recorded.
Shaco — the marionette, now free — is defined by the fact that nobody knows where he came from, what kingdom, what craftsman, what prince. He's been operating for long enough that the original context has been lost, and he has never shown any interest in reconstructing it. He kills. He makes people laugh while he kills them. He leaves "gifts" — boxes that explode into chaos, knives that materialize from nowhere — in places they will cause maximum confusion and suffering.
The uncertainty about whether Shaco is a demon, an enchanted object, a manifestation of collective fear, or simply a very old and very deranged puppet is not incidental. The uncertainty is the point. Shaco has built his entire existence around being unexplainable, because the inexplicable is maximally terrifying, and terror is the art form he has chosen to master. He is completely sincere about this. The jester's grin is not a performance. It is, as best as anyone can tell, just his face.