§ Of the Ruin of Doriath The Silmarillion

Upon Doriath a heavy change had fallen. Melian sat long in silence beside Thingol the King, and her thought passed back into the starlit years and to their first meeting among the nightingales of Nan Elmoth in ages past; and she knew that her parting from Thingol was the forerunner of a greater parting, and that the doom of Doriath was drawing nigh. For Melian was of the divine race of the Valar, and she was a Maia of great power and wisdom; but for love of Elwë Singollo she took upon herself the form of the Elder Children of Ilúvatar, and in that union she became bound by the chain and trammels of the flesh of Arda. In that form she bore to him Lúthien Tinúviel; and in that form she gained a power over the substance of Arda, and by the Girdle of Melian was Doriath defended through long ages from the evils without. But now Thingol lay dead, and his spirit had passed to the halls of Mandos; and with his death a change came also upon Melian. Thus it came to pass that her power was withdrawn in that time from the forests of Neldoreth and Region, and Esgalduin the enchanted river spoke with a different voice, and Doriath lay open to its enemies.