The Black Fog is a large, powerful, and evil Pokémon. It has no qualms with killing others and has been doing so for many years. The Black Fog is apparently very old and has been around since the times where Pokémon were treated as gods. Because of it being accustomed to that treatment, it became arrogant and prideful.
The Black Fog is described as a high-level Haunter that preys on humans and Pokémon and has been doing so for years. It kills its prey by stealing their souls with its Dream Eater and disappears just after it does so.
The Black Fog first appears where Sabrina, its latest victim, is sent to the hospital. One of the people who have been combating the Black Fog tells Ash and Brock that the giant Haunter had killed Sabrina's Pokémon when she was younger. Ash, surprised that a kind woman could be filled with feelings like revenge, decides to defeat the Black Fog and free Sabrina from its grasp.
The statue of a Haunter revealed after its death
After making a large Poké Ball designed to capture the monstrous beast, Ash, Brock, and the people who have been fighting the Black Fog all go to its lair to defeat it. While the group distracts the many Gastly that the Black Fog has to keep surveillance, Ash and Brock stay behind to battle the beast.
They begin battling the Black Fog with their Pokémon but it just easily smacks them around. Eventually, with help from Sabrina and her Abra, they manage to get Black Fog into the Poké Ball and attempt to close it inside. Suddenly, the Poké Ball explodes and the Black Fog escapes into a hole below. They realize that it used Explosion to free itself but in doing so lowered its health down to almost zero and can be captured by a normal Poké Ball.
Ash goes down to battle the Black Fog one last time aided by his Pikachu and Sabrina's Abra. Weakened by using Explosion, Black Fog is forced to defend itself by throwing rocks at its opponent. They easily dodge the rocks and Ash throws an Ultra Ball to capture it after binding Black Fog with Thunder. However, just before the Ultra Ball can reach it, the Black Fog commits suicide with Self-Destruct.
Its death instantly revives Sabrina, who rescues Ash from the collapsing ruins. Sabrina reveals that in the ancient times, Pokémon were once treated as if they were gods and the Black Fog had become accustomed to the treatment so it would rather commit suicide than let itself be caught by a human. Despite hating the beast for over the last ten years, Sabrina cannot help herself from feeling sorry for it and breaks down in tears.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
The Black Fog may be the counterpart to Sabrina's Haunter. It should be noted that the Haunter in the anime was happy and friendly while the Haunter (referred to as "The Black Fog") in The Electric Tale of Pikachu is shown to be evil and destructive; likewise, in The Electric Tale of Pikachu, Sabrina is kind and gentle, while her anime counterpart was cold and callous.
ブラックフォッグ Black Fog
Means Black Fog
For more information on this Pokémon's species, see Haunter.