The High Priest Not to Be Described is the sole occupant of a remote and ancient monastery on the Plateau of Leng in the Dreamlands. He serves the Outer Gods and may be Hastur, the servant of the crawling chaos Nyarlathotep (or he may even be Nyarlathotep himself).
The monastery where he dwells has a confusing tangle of lightless corridors with disturbing frescoes that chronicle Leng's bloodcurdling history. Deep in the bowels of the monastery, inside a frightening domed room, the High Priest Not to Be Described sits on a throne of gold atop a stone dais in pitch-black darkness. Five steps down from the dais is a row of six blood-stained stone altars surrounding a yawning well that is rumored to connect with the Vaults of Zin in the underworld.
In Lovecraft's novella The Dream-Quest of Unknown Kadath (1926), Randolph Carter has a fateful encounter with the High Priest Not to Be Described. The only description of the High Priest is given in this passage:
...and there... sat a lumpish figure robed in yellow silk with red and having a yellow silken mask over its face. To this being the slant-eyed man made certain signs with his hands, and the lurker in the dark replied by raising a disgustingly carven flute of ivory in silk covered paws and blowing certain loathesome sounds from beneath its flowing silken mask.
During his search for the fabled city of Celephaïs, the dreamer Kuranes had an equally chilling experience, just narrowly escaping the High Priest.
Although the identity of the high priest is never established in The Dream-Quest of Unknown Kadath, the following passage (which appears below the passage quoted above) gives a hint:
This colloquy went on for some time, and to Carter there was something sickeningly familiar in the sound of that flute and the stench of the malodorous place. It made him think of a frightful red-litten city and of the revolting procession that once filed through it; of that, and of an awful climb through lunar countryside beyond... Then the figured silk slipped a trifle from one of the greyish-white paws, and Carter knew what the noisome High Priest was.
Since the moon is inhabited by pale, flute-playing toad creatures, and since the High Priest Not to be Described also communicates by playing the flute, he may simply be one of the lunar toad creatures and a servant of Nyarlathotep, the crawling chaos that consistently opposes Randolph Carter throughout the story. Furthermore, when Carter uses an opportune moment to escape and is fleeing blindly through the monastery's darkened, labyrinthine passageways, he dares not think of his pursuer and of "the stealthy padding of shapeless paws on the stones behind him, or of the silent wrigglings and crawlings which must be going on back there in lightless corridors."
He may instead be an avatar of Nyarlathotep, as a similar creature is mentioned in Walter C. DeBill Jr.'s "In 'Ygiroth". The sheer horror of Carter's reaction when he approaches the High Priest in the Yellow Mask lends some support to this possibility.