Apparently he spent some time in the Dreamlands.

“Hei! Aa-shanta 'nygh! You are off! Send back earth's gods to their haunts on unknown Kadath, and pray to all space that you may never meet me in my thousand other forms. Farewell, Randolph Carter, and beware; for I am Nyarlathotep, the Crawling Chaos.”
― H.P. Lovecraft, The Dream-Quest of Unknown Kadath