Iod is a Great Old One worshipped on the planet Bel Yarnak and beyond the farthest galaxies (where he is "The Source"), and is known in Atlantis and Mu (as "The Shining Hunter") who worshipped him as a "god of earth" along with Vorvadoss and Cthulhu. A strange crystalline fungus thing. He is remembered dimly as "the Shining Pursuer, who hunted souls through the Secret Worlds". Most data on him is contained in the Book of Iod.
Iod is a conglomeration of plant, mineral, and animal which appears first as a veil or great black shadow in the air, followed by a blinding night after which it emerges. After paralyzing its victims, Iod drains away their life force with its tendrils until only its brain remains. Which lives on within the corpse, trapped forever.
Call of Cthulhu
In the Call of Cthulhu RPG, Iod is not an Outer God but a Great Old One.