Dark Force was the dark side religion that sprang up around the Dark Force Temple, an ancient Sith temple on Dromund Kaas. It was founded by Darth Millennial, an apostate Sith apprentice who rejected Darth Bane's Rule of Two. The clergymen of the Dark Force religion held the title Prophet of the Dark Side, who viewed the future through the dark side of the Force, attempting to avoid their visions being misconstrued by political agendas or personal ambitions, as so often happened to the Sith.
When Palpatine discovered Dromund Kaas, he annexed the Prophets as his advisors, and appointed the Dark Jedi Kadann as Supreme Prophet of the Dark Side. With the advent of the New Order, he named the Prophets the Emperor's Mages, collectively forming the Secret Order of the Empire. Many of Palpatine's Dark Jedi, Inquisitors, Force-sensitive military officers, and Emperor's Hands received their first exposure to the dark side by studying the Dark Force religion on Dromund Kaas under the tutelage of the Prophets.
Although Palpatine had appointed Kadann to the position of Supreme Prophet, Kadann's true loyalty was not to the New Order, but to the Dark Force. Prior to the Battle of Endor, Kadann's visions indicated a probable restoration of balance to the Force. This prophecy did not settle well with Palpatine, but Kadann refused to compromise, and fled the Emperor's wrath with some of the Mages, while others remained behind with the Emperor (chief among them being Lord Cronal).
Kadann's followers hoped the Dark Force religion could take over the Galaxy. They tried to employ the Dark Jedi Jerec to find the Valley of the Jedi, which the Prophets realized could grant them ultimate knowledge and understanding of all things. Jerec, however, was out only for himself, and in the end, his true objectives were irrelevent, since shortly after discovering the Valley, he was slain by Kyle Katarn. The location of the Valley remained a mystery to the Prophets, and they remained silently in the shadows, waiting for their next opportunity to spread the Dark Force vision.
The Dark Force Temple on Dromund Kaas thus remained abandoned until briefly reoccupied by Kyle Katarn, who was corrupted to the dark side within its walls, but ultimately redeemed by Mara Jade.