Black Fog appears as a Giant Haunter, in the manga it appears with a much spikier appearance than regular hanuter and it's claws and teeth appear sharper. In the anime it has red glowing eyes.
As stated in the Electric Tale of Pikachu Black Fog are able to consume an enemies soul, they are much stronger than Gastly, Haunter, or Gengar and seem to have the power to control other Ghost-types.
In the animeBlack Fog appeared in Fear Factor Phony, Haunter transformed into this form by merging with several other Ghost-types, including two Gastlys, two Duskulls, and a Shuppet.
In the Manga
Electric Tale of Pikachu
Black Fog makes it's debut in the Electric Tale of Pikachu, it appears as a Legendary Pokémon which was worshipped in ancient times. Black Fog killed many of the humans and Pokémon living in Lavendar town until Ash came to capture it, it selfdestructed to avoid capture.