Illuin, also known as Sky Blue, refers to the northern of the two Lamps of the Valar, set on the great tower Helcar by Aulë to bring light to the world of the Valar.

After the Valar entered the world, there was a misty light veiling the barren ground. The Valar took this light and concentrated it into two large lamps, Illuin and Ormal.

Aulë forged two great towers, one in the furthest north, Helcar, and another in the deepest south, Ringol. In the middle of Arda, where the light of the lamps mingled, laid the Great Lake with the island Almaren, where the Valar dwelt.

The lamps were destroyed by an assault by Melkor, and the Valar fled Middle-earth for Valinor. At the site where Illuin fell, the inland Sea of Helcar was formed, of which Cuiviénen was a bay.