Hm, this is interesting, following the search for the elusive sword, and the getting all the stories (currently fully charted both Lapino and Skyripper branches, leaving the Iblis route for last), I came upon an interesting tidbit in one of the Iblis endings.
When Reynardo kills the Councilor and the Emperor by consuming their souls, and Iblis says "It is complete", the Iblis Stone is referred to as "what was once a stone/gem in a sword". Given the look of the eye in that page of the Book, it is most certainly related. Sadly, I don't see how would the Stone end up in a sword in the story, so it seems I'll turn up empty handed, but it is an interesting pursuit either way.

As far as I can tell? technically, yes, it is within the games Files, and I know what it looks like, but as far as if it is fully implemented in the game, not sure, nor do I know how you'd obtain it, I'm gonna *definitely* have to 100% this game now because I wanna see if it was just cut content or something. I was poking around in the game files and found it, then found this thread
Here's a link to what the model looks like. apologies for the weird visual artefacts
https://imgur.com/a/ZuEMvdk

oh i didn't even notice that at first! i hope there's some cool extra weapons or even a way to 'overcharge' the current swords beyond their limits! either way, i'm enjoying every run through this game. :D