The Dark Force religion was brought to Dromund Kaas by the Sith apprentice Darth Millennial. The apprentice abandoned his master to focus on the philosophy of Rule by the Strong, espoused by the "Dark One" Lord Kaan, rather than on Darth Bane's restrictive Rule of Two.
The planet remains a well-kept secret exclusive to the prophets until its discovery by Darth Sidious. It then becomes an early training ground for Palpatine's Force-sensitive minions. In the Old Republic era, heroes may have heard rumors about this dark side religion and seek Dromund Kaas to either destroy or practice the malevolent faith. During the Emperor's reign, adventurers on the planet would have to be darksiders working for Palpatine or Prophets-in-training.
Whether serving the light or darkness, heroes visiting Dromund Kaas will find themselves undergoing grave challenges. Swamp wampas, dianogas, vornskrs, Guardian Spirits, and Noghri abominations all infest the dark side bog world.