Shadowy Isles

A group of islands that lay in the western reaches of the Great Sea, off the eastern shores of Aman. They were shown to Tuor in a vision by Ulmo, and in that vision they appeared in dangerous and stormy seas, battered by immense waves.

The identity of the Shadowy Isles is difficult to assess for sure. In Unfinished Tales, Christopher Tolkien tentatively identifies them with the Enchanted Isles raised at the time of the Hiding of Valinor, and there are certainly many similarities. Both were set in the Shadowy Seas, both lay not far off the eastern coasts of Aman, and both were dangerous to mariners. What's more, it is said that 'the seas about [the Enchanted Isles] were filled with shadows and bewilderment' (Quenta Silmarillion 11, Of the Sun and Moon and the Hiding of Valinor) which would certainly explain 'Shadowy Isles' as an alternative name.

Matters are confused slightly by an early map that shows the Shadowy Isles as somewhat separate from the Enchanted Isles. That map (Map IV in volume IV of The History of Middle-earth) shows the Enchanted Isles as an arc of islands protecting the coasts of Aman, with the Shadowy Isles as an apparently separate group a little beyond the northern end of that arc. In fact, they lie in direct line between Beleriand and Taniquetil, and this matches with Tuor's vision (in which he saw the Holy Mountain rising beyond the Shadowy Isles, just as we would expect from his vantage in Nevrast). Taking all these facts together, the most likely interpretation is probably that the Shadowy Isles were a northern outlier of the entire chain of the Enchanted Isles, clearly connected with the latter, but probably not synonymous with them.