Apocalypse, the name comes from the Greek 'apocalypses', meaning unveiling, revelation, particularly of a divine nature. The faith of the early Christians, living under persecution, was sustained by the expectation of Christ's immenent second coming. This found literary expression in the Revelation of John, written at the end of the first century A.D., an allegory foretelling of the end of the world and the coming of the Last Judgment, the destruction of the wicked, the overthrow of Satan and the establishment of Christ's kingdom on earth.
The sequence of fantastic images - the author's visions - forms a cycle of themes that are found in fine art.
In the book John witnessed that he saw the Lord our God on the throne, in His right hand He had "a scroll with writing on both sides, and sealed with seven seals." (Revelation 5:1). Then the Lamb of all living creatures started to break the seals:
the first seal was broken, and there was "a white horse, and its rider held a bow. He was given a crown, and he rode forth, conquering and to conquer."(Rev. 6:2). It was 'Conquerer' ;
the second seal was broken, "out came another horse, which was red. Its rider was given power to take away peace from the earth that men might slaughter one another; and he was given a great sword." (Rev.6:4). It was 'War';
the third seal was broken, and there came the black horse, "and its rider was holding in his hand a pair of scales", he was ordered to take away crops" (Rev.6:5). It was 'Famine';
the forth seal was broken, and "there was another horse, sickly pale; its rider's name was Death, and Hades followed close behind." (Rev.6:8).
These four are called the Horsemen of the Apocalypse and usually interpreted as the agents of divine rage.
See: Dürer The Four Horsemen of the Apocalypse.
Russian Icon. The Apocalypse.
Luca Signorelli. The End of the World, Apocalypse.