The Shadowy Seas were that part of the Great Sea that lay closest to the shores of the Blessed Realm, to the immediate east of Tol Eressëa and the Bay of Eldamar. After the Exile of the Noldor and the first rising of the Moon, the Valar elected to shield their realm from the outside world. They enmeshed these seas with a bewildering shadow - the source of their name - and created a chain of Enchanted Isles, each of which had the power to ensnare any mariner braving this region. Throughout the centuries that followed, no ship reached the shores of Aman, until Eärendil finally succeeded in passing through the shadows of the Shadowy Seas.
In other stories
The Shadowy Seas are referenced in Tolkien's Roverandom. In the story, written down in 1927, the great whale Uin takes the enchanted dog Roverandom (formerly known as Rover) on adventures through the seas: Uin takes Roverandom through the Shadowy Seas to the Bay of Fairyland beyond the Magic Isles where Rover saw the Mountains of Elvenhome and the light of Faery. Roverandom thought he could see the white glint of a city of Elves on a green hill far away in the distance.
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Quenta Silmarillion: Of the Sun and Moon and the Hiding of Valinor"
- J.R.R. Tolkien, Christopher Tolkien (ed.), The Silmarillion, "Quenta Silmarillion: Of the Voyage of Eärendil and the War of Wrath"
- J.R.R. Tolkien; Christina Scull, Wayne G. Hammond (eds.), Roverandom, pp. 73-4