§ Of Beren and Lúthien The Silmarillion

In the time when Sauron cast Beren into the pit a weight of horror came upon Lúthien's heart; and going to Melian for counsel she learned that Beren lay in the dungeons of Tol-in-Gaurhoth without hope of rescue. Then Lúthien, perceiving that no help would come from any other on earth, resolved to fly from Doriath and come herself to him; but she sought the aid of Daeron, and he betrayed her purpose because he would not deprive Lúthien of the lights of heaven, lest she fail and fade, and yet would restrain her, he caused a house to be built from which she should not escape. Not far from the gates of Menegroth stood the greatest of all the trees in the Forest of Neldoreth; and that was a beech-forest and the northern half of the kingdom. This mighty beech was named Hírilorn, and it had three trunks, equal in girth, smooth in rind, and exceeding tall; no branches grew from them for a great height above the ground. Far aloft between the shafts of Hírilorn a wooden house was built, and there Lúthien was made to dwell; and ladders were taken away and guarded, save only when the servants of Thingol wrought her such things as she needed.