Exar Kun
He's from the 'Knights of the Old Republic' comic books made way back when! somewhere in these forums we had a descussion on him and Nur Ab Sal managed to post up an alarming number of screenshots from the series.
Anyway, he got betrayed by Ulic-quel Droma and ended up becoming a sort of living spirit and he inhabited the Yavin IV Jedi Academy where he started to possess certain Jedi traniee's, but was finally put down during that little story line.
Lasted 4000 years though, a feet no Sith Lord has managed before or since, but then that's only if you like the EU, the others will simply tell you 'That's not Canon'
Canon, what a stupid word.