The High Priest Not to Be Described is a minor character in The Dream Quest of Unknown Kadath by HP Lovecraft. Little is known about the priest as it is encountered only once in the story, and the protagonist, Randolph Carter, fearful of the potential dangers, flees soon after meeting the creature.
In The Dream Quest of Unknown Kadath, Carter is captured and taken by shantak-bird over the feared Plateau of Leng. Far past there deep inside a labyrinthine tunnel system, he is forced into an audience with the High Priest Not to Be Described. Sensing how truly dangerous this thing is and gripped with a wild fear, Carter knocks one of his captors into a well-like aperture in the cave floor and flees with no plan of escape. This is the last thing we see of the High Priest Not to Be Described.
We consequently know very little about this creature. We know it is clad in a yellow robe and dwells deep in Earth's Dreamlands, served by the moon beasts, shantak birds, and the Men of Leng. The yellow could suggest connections to Hastur, the unspeakable, but since August Delrith is credited with the connection between the King in Yellow and Hastur after this story was written, it is likely a coincidence. More probable is that this is some aspect of Nyarlathotep meant to deal with dangerous dreamers. It should be noted, however, that Nyarlathotep does appear elsewhere in the story in its pharaoh form.