A great dreamer is one who who is able to view through the thin wall between worlds between the Other World (Earth) and the World of Daventry in their sleep.
Background
Roberta Williams is the great dreamer, and it was her dreams that allowed her to create the King's Quest games based on the history of world of Daventry. Daventry may be a place one can only reach through the dreams of the dreamer Roberta Williams, creator of the games themselves.[1]
Derek knew that a great dreamer in the other world created tales and fables from that which was Daventry's own reality, Daventry's own history. He also believed that there may also be another great dreamer now living within his world, who could dream of the fictions of the other universe, and was creating new realities and histories for his world based on them. He believed the worlds were drawing together, they had already been fairly close to begin with. That both worlds dream each other. The other world was creating its own visions of Derek's world, a different Daventry than his people inhabit. And now his people were dreaming of the version told in the Other World, and with that his world changed.
- ↑ KQC4E, pg315