King Dark is a giant robot who served as the figurehead of the Government of Darkness.
He reveals himself after Apollo Geist's destruction, mocking Kamen Rider X. Destroyed by a self-destruct function that its pilot Dr. Noroi activated before he died. He is permanently giant-sized, and is usually found in the pictured position in order to command his human-sized underlings.
All Riders vs. Dai-Shocker
King Dark appears as a member of Dai-Shocker, menacing the Kamen Rider. However Diend arrived with Kamen Rider J. Using the Decadriver Final FormRide card, he turned Decade into the Jumbo Decadriver which then turns J into Decade Complete Form Jumbo Formation and he destroys King Dark with a variation of the Dimension Kick, using the other Riders as giant Kamen Ride Cards.
Let's Go Kamen Riders
Due to a change in history caused by Ankh leaving a O Medal behind in the year 1971, Shocker achieved its goal of world dominion with the Shocker Greeed and new members from the various groups that would had manifested in the normal timeline after Shocker's destruction. King Dark, Apollo Geist and the Government of Darkness were part of Shocker's alliance. They were destroyed when the Great Leader became the Great Rock Leader.