Formenos was the fortress that Feanor built in the north of Valinor, whither he withdrew with his sons and his father Finwe after he was banished from Tirion for threatening his half-brother. There the Silmarils were kept in a chamber of iron, and a great hoard of gems was gathered. Thither came Morgoth in the dark after the slaying of the Two Trees: he slew Finwe upon the threshold -- the first death of the Eldar in Aman -- and carried off the Silmarils, and that deed kindled the rebellion of the Noldor.