There is an ancient book of prophecies, held deep within the archives of the Citadel. The Prophecies of the First Men, it is titled, and that is what it holds.
Of course there are many books of prophecy in the citadel. Aside some few copies of the words of Daena the Dreamer or other Targaryen dragon dreamers, there are many which carry the words of woodswitches and others purported to have the gift of prophecy. The difference between this book and those is that every prophecy set in the first three-hundred pages has come true.
The unnamed seer saw all which was to come, from the invasion of the Andals to Robert's Rebellion, and even fortold of the plague which would take that Baratheon King's life. But among the many prophecies one stands apart from others in its brevity and boldness:
THE WALL SHALL FALL. HUMANITY SHALL BE PRESERVED BY THE SONG OF ICE AND FIRE.
Of course, ancient books such as this one are kept in the most secure and safe of environments. The book read by any who saw these words was a copy of a copy of a copy. As often comes to pass when one is transcribing an ancient, frail tome, the scribe misread and miswrote some words here and there. Crucially, he misread this word - 'Song' should, in fact, have been 'Sons'.