Rivan was a Zelosian, Ragnos's rule details revealed, Exar Kun did NOT invent the DBL
From the Essential guide to the Force, lot of details revealed. The sith section actually comes from...Palpatine's very own holocron and contains excellent images and info, including Palpatine being trained by Plagueis as a young man.
Now, for Ragnos: Palpatine details the records of Ragnos as extremely powerful and praises him for his caution. (Palpatine almost outright calls Naga Sadow an idiot at several points for a few reasons)
Ragnos's rule details: He united the Sith Lords in a quick series of campaigns in battle and stayed in power by manipulating his rivals against one another, manipulating the weaker ones into challenging him, or simply having the stronger ones assassinated. His cause of death is mentioned as 'possibly old age.'
Palpatine is mentioned as having gained all of Naga Sadow's knowledge from an old Sith holocron and comments Naga was far too generous with it...he even details 'some' of Naga's knowledge in "The Dark Side Compendium" Palpatine gives accounts from Dark Lords from the Ancient Empire, the NEw Sith Empire, primarily Seviss Vaa, one of Kaan's stronger Sith Lords-Palp also holds some of his info in skepticism as Palpatine proved him wrong in several areas. And Kla, a Jedi master and scholar turned Sith Lord. Palpatine remarks one thing Sith can learn from Jedi is not being brash morons (Like Naga Sadow. Palpatine REALLY doesn't like him)
And he said though he's sometimes criticized for his 'inaction' during the early events, he says he ignores such critics as things proceeded directly as he wanted. He already had his eye on Dooku and Maul achieved his purpose perfectly.
And perhaps largest of all: Exar Kun is confirmed to...NOT have invented the DBL. Sorry, Exar fans, but it's mentioned he learned the design from studying a Sith holocron containing information from the first Jedi exiles.
Some nice stuff with Ajunta Pall, too...and first hand accounts from several Sith. Malak, btw, is confirmed as 'Alek' from the KOTOR comics
A lot of illustrations are VERY well done...aforementioned Palp and Plagueis, Palp and Maul, Mara Jade's funeral (OK, a few complaints on that one...Luke looks too happy in that picture, but Ben and Jacen look great.)
All in all, damn fine guide