The next person who took on the title of being the Dark Lord of the Sith after Exar Kun was Darth Revan. He was a Jedi who fought in the Mandalorian Wars. After defeating Mandalore the Ultimate, he traveled to the Unknown Regions with his apprentice, Malak, and they came back as Sith Lords. If you want to read more about Revan, you can check out a full bio of him here.