Nothing more than a speculation and maybe a theory, but Fiddle's and Swain's Demon seem AWFULLY similar. Considering they are both demons, and we already know that Tahmn and Eve are demons. It just seems weird that Fiddle's and Swain's demon share a very similar design. The shapeless black with red trim, while among the black are tons of red eyes. They also both share a bird-like design, Fiddle's has "feathers" sprouting from his back (more noticeable in the in-game model) and Swain's obvious wings. They also bring birds to them. Lastly, they share a similar sense of immense destructive nature and capabilities.
With all that, i like to think they have the same TYPE of demon or possibly related. Which hints that demons come with categories and types. I know this last bit won't come true because Riot won't kill off characters but it would be sooooo COOL if Fiddle's demon is Swain's at different period of times. Like Swain obtained Fiddle's demon through "destroying" Fiddles. This really all came to mind while researching Fiddles while working on a one-shot for my D&D/League conversion.