Darkening+of+Valinor

The Darkening of Valinor is the destruction of the Two Trees of Valinor, it was 50 years before the begining of the first age and was done by Morgoth and Ungoliant. These two trees were Telperion (silver) and Laurelin (gold). They brought 7 hours of light to Valar everyday. Without this light all of Valar becomes dark and vulnerable.

http://en.wikipedia.org/wiki/Darkening_of_Valinor