Here's a thought on how to retcon Shaco using inspiration from Majora's Mask
Shaco's one of those Champions who could have an interesting background, but is too clouded right now by ambiguity, much like the rest of League's current lore (BA-ZING!). Having just done a full playthrough of Majora's Mask 3D (which is an AWESOME remake of the original with its reworked boss fights), I was given an idea on how to retcon our Demon Jester.
This is just a rough synopsis, so take it with a grain of salt. It's meant to just give the gist of the idea. (UPDATED!)
Shaco was once a wiry Noxian cutpurse who learned of a noble who had come into possession of an ancient mask. Shaco, believing he could make a fortune fencing the mask, disguised himself as a jester and snuck into the noble's estate to steal the mask, but did not know the mask had a terrible curse upon it. Caught in the act, Shaco was struck down by the noble's guards, but in his dying breath, he placed the mask on his face, allowing it to possess him.
Shaco was then transformed into a demonic creature, a sadistic jester who would laugh at the suffering and death of others. With his newfound power, Shaco made short work of the noble's guards, and then tormented the noble, who soon killed himself from insanity.
The story goes from there. It's a tad akin to Varus, but not in the sense that Shaco wanted revenge for anything. It'd be a bit more like a mix between The Joker's origin and Majora's Mask.
Does this seem like a good starting point?
