At some point before 3954 BBY, the Sith Empire conquered Drezzi, and the husband of Drezzi's Queen Ressa was killed by the Dark Councilor Darth Nyriss. In gratitude for Nyriss' decision to spare her life and those of her children, Ressa sent a decorative holoprojection to the Sith Lord that was displayed in Nyriss's residence.
--Wookiepedia
 
The reconstituted Sith Empire arrived to find a world of Force users that called themselves Sith, but had culturally drifted over the generations since the Great Hyperspace War. After determining that the world was still abundant with resources—even if the current inhabitants could not exploit them—, and that the population could be easily integrated into the Sith Empire, Darth Nyriss strode into the throne room of Drezzi’s most powerful kingdom, Ty’azai. Her terms were direct and simple: swear loyalty to the Sith Empire, or face a forceful conquest and be brought to their knees by orbital bombardment.
 
The king of Ty’azai, Daar’tah Tramere, was appalled by the idea of anyone claiming the name of the “Sith Empire” and was unwilling to answer to any authority over his own. His flat refusal of Nyriss’s terms resulted in swift action, and a violent conflict immediately ensued within the Ty’azai fortress; though the king fought well, the powerful Nyriss single-handedly slew him, the fortress’s defenders, and anyone daring to challenge her. Queen Ressa and her children, thanks to having retreated to the fortress’s crypts, were the last remaining members of the royal house’s bloodline. Nyriss approached her and again offered her terms; the queen gratefully accepted.

Following Queen Ressa’s surrender, House Ty’azai began their conquest Drezzi in a grand war of unification. The Sith Empire provided considerable support to Ty’azai, leading to an easy domination. Within five months all the lords of Drezzi swore oaths of loyalty and vassalage to the Sith Empire and House Ty’azai, respectively. The Kingdom of Drezzi—or simply, “the Kingdom”—was established. When the short conflict was settled, Drezzi was part of the Sith Empire once again, and for the first time a single sentient ruled the planet.