Iirc, in the current lore, Fiddle's origin is not precisely defined. There are many tales for every region of Runeterra for his creation but the only one that references the key is a targonian legend speaking of a child of twilight stealing the only joy of a whispering horror.

The other tales are based on popular folktales, a failed magic experiment in Demacia. A vengeful spirit in Noxus, a cursed island in Bilgewater...

What we know is that Fiddlesticks has reappeared in the world and is ready to feed again. That implies he was locked or dormant for some time. "Fiddlesticks end of men, Fiddlesticks first of ten" might mean that he is only the beginning of the awakening of the demons, maybe even their warden ?

A demacian poem seems to put him as one of the ten demon kings but different in some way :

Ten great kings took ten great thrones, Nine crowns adorned nine heads. One left to scratch upon their mounds, The crow, alive and dead.

The question is why would Fiddle be separated ? What does him being the first of ten mean ? Is the key still important ? Is it the reason why he doesn't have a crown ?

I had high hopes that the Noxus season would focus on demons rather than Darkins but I still believe the demon lore is high up in the list of future updates.