However, either the collected wisdom of twitter, or the pushing the question to the side of my brain worked, and I knew the correct answer. So here, from one of the far too many works in progress, That Sweet Sleep of Death, is the scene, and the answer:
Two gates guard the Temple of Dreams, one carved from ivory, and the other from horn. It's easy to think that the division is illusion and truth, for the stories have it so. Many newcomers to the City of Nyx make that mistake.
But the divide between the gates isn't quite that simple. Rather, dreams of deception pass through the Gate of Ivory and dreams of fulfillment through the Gate of Horn. Never doubt, as you choose which gate to walk through, that there is more to deception than mere illusion, and that more than truth is required for fulfillment.
I am a psychopomp. The dreams in my aegis are the dreams of fulfillment in a very absolute sense of the word. When I go to the Temple on business, the gate I walk through is the Gate of Horn.