We don't know much about most of them, save for their lingering deeds. Darth Kaan was responsible for the near-total destruction of the first Sith order, detonating a superweapon in a desperate move on the planet Ruusan. Darth Bane, his immediate successor, established the core defining law of the Sith. The "Rule of Two". One master, one Apprentice. No more, and no less. Darth Krayt rescinded the rule of two, swelling the ranks of the Sith for a second war against the Jedi Plageus is recorded as attempting to manipulate the midichlorians in an attempt to create or restore life, actions that are speculated to have led to the birth of Anakin Skywalker. Sidious, Maul, Tyrannus, and Vader are all well-known figures. You know about these ones.