• Adventures: All That Glitters, Holiday in the Sun, Adventus Regis, Herald of the Yellow King, Death in Freeport, Madness in Freeport, Terror in Freeport
• Race: Great Old Ones
• Description: The Unspeakable One appears as the King in Yellow: The King has no face and is twice as tall as a man. It wears pointed shoes under its tattered, yellow robes, and a streamer of silk appears to fall from the pointed tip of its hood. It morphs and changes, at times winged, at others, haloed.

Despite its title, the Unspeakable One has many names. The elorii call him Umor. The human texts refer to the Other. In some texts it is the King in Yellow. But perhaps "Unspeakable One" is apt, as all references to this being have been stricken from human records.

Whatever its name, the Unspeakable One is known to have corrupted mankind somehow. When the gods gave chase, it fled to the elorii lands. To defeat it, the human gods (known as Great Ones) destroyed the elorii gods (known as Elder Gods) and absorbed their power, thereby giving the Great Ones the ability to imprison the Unspeakable One in his tower. That tower is believed to be on the dark moon, sometimes called Kadath, sometimes called Hastur. It is in this tower, which is in the city of Carcosa, that the Unspeakable One waits, restless, plotting its return.

The Unspeakable One is worshipped by the Brotherhood of the Yellow Sign. They seek to bring the Great Old Ones back and destroy civilization, with the hopes that it will rebuild anew. Thus the Valossan ssanu strive to return to their ancient glory, while the insane human cultists who worship the Unspeakable One truly long for armageddon.

When the Unspeakable One appears, it is as the King in Yellow, a horrific figure from a play by the same name. It has manifested itself twice so far; the first time, the entire town of Vestalanium went mad. The second time, all of Moratavia nearly disappeared.