In the 14th century, Ventax II was a very different place. There was a great deal of pollution on the planet, a number of city-states warred continuously, and the populace was not very well off. At the height of the crisis, a council met to assess the Ventaxian people's options. The Ventaxians switched from an industrial to an agrarian based economy. A number of initiatives were undertaken to reduce pollution and to clean up the environment.
It's believed that a Ventaxian leader or group of leaders composed a contract with Ardra, which promised 1,000 years of peace and prosperity in exchange for Ardra taking possession of the planet at the end of the term. It became a fundamental theology of the planet, and Ardra became the Ventaxian version of the devil.
Up until the 24th century, the Ventaxians were a peaceful and prosperous people. A Klingon expedition made first contact with them in the 23rd century, but the species did not become too involved in the galactic community. While modern technology was available, Ventaxians generally were not interested in it.
In the 2360s, a female con artist became aware of the legend of Ardra, and decided to use it to exploit the Ventaxians. She developed a system of holography, transporter effects, and tractor beams to create a convincing "Ardra" - who appeared exactly when the contract said Ardra would appear. Captain Picard of the USS Enterprise (NCC-1701-D) helped to expose the con artist, and Data (as arbitrator) ruled that since Ardra had withdrawn from the contract, that it was null and void. (TNG episode: "Devil's Due")