Located in western Minhiriath, Eryn Vorn was originally part of the vast ancient treescape that covered most of north-western Middle-earth. It was named 'The Black Wood' by the Númenóreans during the second age.
During the Second Age, however, these forests were demolished by the Númenóreans in their greed for shipbuilding timber, before being almost completely burnt down by the forces of Sauron during the ensuing war in Eriador.
By the latter half of the Second Age, the surviving natives of Minhiriath had retreated either north to Bree, or hidden themselves in Eryn Vorn. Eryn Vorn was probably all that remained of the vast forests in Minhiriath afterwards, but by the end of the Third Age, scattered woodlands had reappeared in much of the rest of Minhiriath.
From TA 861, Eryn Vorn nominally formed a part of Cardolan (Arnor), but was never really under the control of the king. It is probable that after Arnor's fall, people remained hidden in Eryn Vorn, for although it is clearly recorded that no permanent settlements of men existed anywhere west of Bree by the late Third Age, it is said in Unfinished Tales that "a few secretive hunter-folk lived in the woods" of Minhiriath at the time of War of the Ring.