When the waters became calm, Ys of the Ages, Ys the Beautiful, Ys of the Many Palaces, was sunk beneath the sea. In later times, when the light was right and the water clear, fishermen sometimes glimpsed the wonderful structures of marble, where nothing moved but schools of fish.
If you haven't read Jack Vance's Lyonesse trilogy then, really, what are you doing here? Vance was a huge influence on both fantasy fiction and fantasy roleplaying, the Lyonesse books are a delight worth returning to again and again, and in point of fact they'd make a great open-world gamebook in the Fabled Lands style.
I don't have the rights, so we're plumb out of luck as far as the gamebook goes, but Lawrence Whitaker of The Design Mechanism has just published a Lyonesse RPG based on Mythras, the current incarnation of the game formerly known as Runequest. If the video whets your appetite, you can buy it on DriveThruRPG. (Full disclosure: I was one of the writing team and penned the sections on Ys and the Ska. But I'd be recommending it anyway, because it's Vance.)
The new RPG offers an exhaustive (500-page) guide to the world, but what if Mythras/RQ isn't your beaker of the blushful? Once you're read the Lyonesse books you'll no doubt have your own take on how to run it. There was another set of roleplaying rules published in 1999 by Men in Cheese, If you want to focus on the activities of sorcerers and their associates, Lyonesse would be a great fit with Ars Magica, I'd love to see a Powered By The Apocalypse version, and you might even use Dragon Warriors. Whatever your favourite system, just dive in. It's a world that is guaranteed to catalyze your creativity.
'I could cite other such relics. Those which are not lost are revered and guarded with care. They might be difficult to obtain.’
Queen Sollace spoke decisively: ‘No good thing comes without hardship. That is the lesson of life!’
‘How true,’ intoned Father Umphred. ‘Your Highness has succinctly clarified a whole heron’s nest of untidy ambiguities.’