Ivory Gate Of Dreams

Graeco-Roman: when a dreamer leaves the realm of dreams, there are two gates he can use to exit to the waking world (though it's usually not up to the dreamer to decide which gate to take). If s/he exits through the gate of horn the dreamer will remember the dream, which will be true. If s/he exits through the gate of ivory, the dreamer will not remember, and the dream will be false.