The Dreamlands first appeared in the story The Silver Key (H.P. Lovecraft, 1926).
The Dreamlsnds are an alternate dimension accessible through a person's dreams. Earlier in life, most people can enter the Dreamlands at will, but as adulthood approaches, this gateway closes for the majority of these dreamers.
The influence of the Great Old Ones in the Dreamlands is minimal, though these beings do possess some power over this realm. Out of all the Mythos, Nyarlathotep holds the most power in this land. The main deities of the Dreamlands are the gods of earth, or the Great Ones, who are weak beings that mortals may overcome or outwit, yet who do take part in mortal affairs on rare occasions. These gods used to dance on the highest peaks of the Dreamlands, but as humans began to climb their beloved mountains, they withdrew to their home in Kadath in the Cold Waste, to be ruled by Nyarlathotep. Only Randolph Carter ever ascended Kadath, and his journey was fraught with the greatest perils.