The Fog are a group of alien monsters from the 1994 film Kamen Rider J. This parasitic alien race's very presence is signaled by a mass amount of mist and cataclysmic events. Every millennium, the Fog Mother gives birth to a new brood to feed on the life forms of a planet she landed on. One such brood was responsible for the mass extinction of the Earth's dinosaurs, which were their sole food supply. 75 million years later, the Fog Mother returns to Earth with a new brood while her three children perform the ritualistic prayer before going out to obtain the sacrifice for their siblings to feed on so they may consume the human race.
Their attempt failed however, due to the Spirits of Earth choosing Kouji Segawa to become a champion of Earth's lifeforms as Kamen Rider J, who destroyed them and the Fog Mother with his powers.
However, the fate of the previous broods that were successfully born are unknown. This leads to an open-ended possibility that the Fog could return in future installments of the Kamen Rider franchise like with many villains in the series.