Star Wars: 10 Most Powerful Sith Lords
4. Exar Kun - The Galaxy Threatener
When Darth Vader stated "the ability to destroy a planet is insignificant next to the power of the Force" he may have been referring to Exar Kun, another fallen Jedi who likewise attempt to enslave the galaxy, only a few thousand years before. Kun was already a powerful Force user before turning to the Dark, but after coming into contact with Freedon Nadd's (Naga Sadow's killer for those keeping up) Force ghost grew even stronger; the ancient lord at first tried to control his new "apprentice", but Exar tricked him, beating out the spirit and learning even more as a result (it's this that keeps Nadd off the list, despite his earlier successes).
That's classic Sith ascension, and some of his actions at the top are just diabolical, including tricking one of Nadd's followers into causing a planet-engulfing supernova. To top it all off, upon death he released his spirit from his body and, despite being trapped on Yavin IV (which like Ragnos caused a bit of a problem for Luke in the Legends canon), thus lived on for millennia.
He also invented the double-bladed lightsaber, which gives him a few automatic badass points.