"had to reach the gates of horn and ivory"

The gates of horn and ivory are a literary image used to show whether dreams are of factual or false events. Because of the similarity between the words "horn" to "fulfill" and "ivory" to "deceive" in ancient Greek, true dreams were said to come through the gates of horn and false dreams to come through the gates of ivory.

The expression dates from Homer's time, its first use being in book 19 of The Odyssey.

The Odyssey on Book Drum