Cliffs rise steeply from the lake edge up to the summits of Garnedd Uchaf and Foel Grach, giving it a dark brooding appearance - hence its name.

The outflow from the lake, the Afon Dulyn, also feeds water into neighbouring Llyn Eigiau.

"Back in Wales, the search for symbols of conquest and the celebration of victory continued in a similarly Arthurian vein, For most of June, including his forty-fifth birthday, the king chose to keep his court at Llyn Cwm Dulyn, a deep, dark lake in the mountains to the south of Caernarfon, reputed to have mystical properties.

"[3] That lake, with a similar name, should not be confused with Llyn Dulyn, which is located more than fifteen miles as the crow flies northeast.

[4] It is also said that if a person stands at the edge of the lake on one of the three "spirit nights" (including Halloween), then he or she will see images in the water of those who will die within the year.